[Elise] is one AWESOME damage dealer, if you know how to play with her.
She might look OP to people who see her in the shop, and she IS OP, but very hard. The combination of harassing from far, minion slaining and the close combat with her Spider Form / Human Form is very hard to control!
After you success to know and controll her, the sky are the limit!

Pros:
* Once you controlled Elise, you can easily win the mid-lane.
* Elise have 4 damagin-skills! (great for brust damage!!)
* Elise have one AWESOME skill ( Cocoon / Rappel) that help you easily focus on the squishiest target on teamfights (usually AD carry, AP carry or Teemo), and ofcurse run away from ganks.
* There is no AP midder who can count Volatile Spiderling / Skittering Frenzy on her spider form if it done right.
* She have healing effect from her Volatile Spiderling / Skittering Frenzy on spider form.
* None of Elise's abilitis of her spider form cost mana.

Cons:
* Elise is very, very, VERY hard to control.
* It realy hard to all of the sudden switch mode between the range-made champion and the close-combat champion.
* Her only CC skill ( Cocoon / Rappel) is very hard to land.
* Squishy AP carry.

Nothing realy to explain...
I recommend the most offensive rune build (9 - Greater Seal of Scaling Ability Power, 9 - Greater Glyph of Scaling Ability Power, 9 - Greater Mark of Magic Penetration, 3 - Greater Quintessence of Ability Power), but for that you need to be sure you can survive the early-game and to know you won't waste all you mana befor 5 min...
I you know you can't handle it or you just want to be more sure, you can use 9 - Greater Glyph of Warding for magic resist and/or 4 - Greater Seal of Replenishment & 5 - Greater Seal of Armor, for armor and mana reg.
Your choice fellows!

STARTER ITEM:

For the starter item I see only 2 optional choices:

Doran's Ring:
Doran's Ring is sure good for beginners. It bring nice start boost of AP, health and mana reg.
Usually, when you buy Doran's Ring, I recommend to buy couple. This way you gain extra damage and extra survivability for the begining.


Boots:
Boots is the more popular starter item with the higher levels. Also, in my opinion, Boots is a better started item from some reasons:
1. Speed at the begining of the game is very important, because the enemy champion can't kill you in 1 or 2 hits, and you have more opportunities to run and survive, OR to reach the enemy and give him some more hits befor he run behind the turret.
2. Buying Boots leave you with 135 Gold, which let you buy extra 3 Health Potion. those 3 Health Potion will be the best thing for survivability that you ever have in the first minutes of the game.

Early-Game Items:

For early-game, I recommend 3 build options:

1. 2 Doran's Ring and Boots - No metter what was your first item, you will always buy those three items, and after you will be able to upgrade your shoes and start to buy the mid-game items. This items combination are sure do safe for beginners, but they are slow you down in your way to get your main items. I use this build for many AP-midder champions (as Ahri for example), but not to Elise, because of her great mobility and survivability potentail, even befor level 6.


2. Boots & Blasting Wand - only if you started with the Boots! This early-game items are more offensivly and will advanced you to the Rod of Ages. Although you might consider with yourself if you want to risk yourself with NO survivability items at all! BUT, if you can hanlde with this condition, Blasting Wand is a better option.

catalyst the protector
3. Boots & catalyst the protector - only if you started with the Boots! This items combination, is the least offensivly one, BUT it is the one that will promise you a well survivability start AND will advence you as well to the Rod of Ages. It is recommended for you if you fight against AP-midder, and you can tell for sure he is eating you alive! If this is the situation, you should buy catalyst the protector right after the Boots (ofcurse beging with the Ruby Crystal). Although, and with all the respect, I don't recommend this build, because if you find your lane hard, it is more recommended to buy 2 Doran's Ring and get the boost of health and AP, and AFTER that buy catalyst the protector.


4. Boots & Needlessly Large Rod - ok, I know. "NEEDLESSLY?! DID HE GET NUTS?!" so, no! I didn't! If you can see that your lane is easy to handle, you don't afraid from your opponent, you farm as hell, don't get into sticky situations, harass for fun and then you can buy Needlessly Large Rod for second item, and get so close to your Rabadon's Deathcap. Personality, this is my favortie build. The only thing is that you all need to think with yourselves if you sure you can handle with NOT a single survivability item only untill you bought Rabadon's Deathcap & Sorcerer's Shoes.

mid-game items:

Here, we all level 6, and the gaming level is incresing.
Now, after we have 2-3 starter items, we can start to get our shoes and first complete item.

Shoes:

Sorcerer's Shoes. Done.
And now seriusly, the first thing you do after yoy buy your early-game items is to buy shoes.
THAT'S ALL.

First complete item (not include shoes):

For the first complete item we have only 2 options:

1. Rod of Ages. the most basic first complete item: give you syrvivability, as well as nice AP boost (80 AP after all of the stacks are complete - equal to Needlessly Large Rod AP bonus).
If you bought for early-game items Blasting Wand/ catalyst the protector, you next move will be Rod of Ages or you will die! (and not in the game... I will find you, and KILL YOU!... jk). More than that, if you will buy 2 Doran's Ring, you will might consider Rod of Ages if you see it is hard for you to keep survive at the mid.


2. Rabadon's Deathcap. The best choice you got is get Rabadon's Deathcap as soon as you can. If you started with Needlessly Large Rod you are in a good position. if you didn't, and you see you can handle your lane with not much survivability, you can buy Rabadon's Deathcap ever if you didn't complete Rod of Ages, and after that, you can back to complete your Rod of Ages.

It is important to tell that no metter what is your early-game items or first complete item is, you have to have Rabadon's Deathcap, Rod of Ages and Sorcerer's Shoes as 3 first items.

Late-Game Builds:

Now we left 3 empty places. Here are your options to fill them:


1. Lich Bane - Good for Elise because of her brust damgae and multi abilities.


2. Rylai's Crystal Scepter - give you NICE bonus of health and the AWESOME ability to slow enemies.


3. Will of the Ancients - Spellvamp. Speak for himself.


4. Void Staff - recommended only if you fight against a magic resist tank (e.g. Galio). If you dont, there are others items that better to Elise.


5. Zhonya's Hourglass - Gives you nice bonus of AP and armor, but not recomended, because Elise has it passive in her ability Cocoon / Rappel (on her spider form), so... you can have better item...


6. Abyssal Mask - A great item for Elise. Abyssal Mask acually works to each champion that is in sitiuation of close-combat with other champion, thanks to its passive. In that case, if you play good with both forms of Elise, this is a MUST item. If you don't, buy other one.


7. Archangel's Staff/ Athene's Unholy Grail - Um... not my choice... they are both can be good on Elise, but they don't have the potential that other items have on Elise. Try other items.


8. Guardian Angel/ Frozen Heart - If you need desspertly survivability, you should buy one of those, but if you are getting handle, buy AP items.

Passive - spider swarm:

Elise passive works on stacks: Every time Elise attacks an enemy unite, she get 1 Spider Queen stack, untill she gets to the max. When Elise change to her spider form, little Spiderlings are summoning. For every stack of Spider Queen summon 1 spiderling. The stacks are fixed, although, if one of your spiderling is dead, the number of the Spider Queen stacks will decrese for every spiderling who was killed, and it will incresed back when you will hit again enemy unit (and so on, and so on...).

Q - NEUROTOXIN / VENOMOUS BITE:

I will begin with the Neurotoxin ( Elise's Q on human form).
This ability is a direct attack (you select the enemy and it automaticly hit the target) so you don't have much chances to miss it. This ability has base damage and it works about enemy's health%.
On Elise's spider form, Venomous Bite works the same with the damage it deals, only when you target an enemy unit, Elise jumps on it besides deals the damage.
This ability (Venomous Bite) sure don't let you miss the target, but you need to consider the use on it. If the target runs to behind her turret, or running to her team, you would prefare to check if you are in a good posission to turret dive (most of the times, YOU ARE NOT!), or get into the enemy team (Just don't... ever! unless your team is coming with you, and even when that happens, you should check more things, I will tell when we reach to gameplay). But beside this conditions, this ability is great if you chase enemy with low health, or if you run from one and reach to wild monion or enemy minion and escape.

W - VOLATILE SPIDERLING / SKITTERING FRENZY:

On her human form, Volatile Spiderling / Skittering Frenzy release spiderling that chasing the closest enemy champion and explode on him. Although, notice that this spiderling will chase the enemy champion, BUT will explode on the first thing it will hit, so when you release it, try to make him go around the minions. Once you mastered aiming with this ability, you will become VERY strong, because of the high damage this ability deals.
On her spider form, Volatile Spiderling / Skittering Frenzy gives you and your spiderlings passive attack speed. When you active it, you get even more attack speed and every hit of you or your spiderlings heals you with tiny amount of health based on you AP. Once you mastered that ability, you may consider yourself the winner of your lane. no metter what champion is fighting you (well... most of the times...). There is no AP-midder who can count 3 AP-damage hits & this ability.
You should know that if you controll this ability (on both forms) you can give it the first point an max it first, but usually it is recommended to max first Neurotoxin / Venomous Bite.

E - COCOON / RAPPEL:

On Elise human form, by using Cocoon / Rappel, you throw spider's webs that stun the first target it lands on for 1.5 seconds. You should now how to aim it right, but beside that, it is a great CC ability.
On Elise spider form, by using Cocoon / Rappel, you hanging on a spider web which probebly sticked to the sky (or some other invisable roof), and become untouchable for 2 seconds. While you are in the air, you can jump on any enemy unit in a big range, although it is important to remember, THIS ABILITY DON'T HAVE DAMAGE! One time I forgot that piece of information, got so exited for catching Rengar with 8 health left(!!!), and forgot to land a basic attack on him, and he ran away (well, after that I released Volatile Spiderling and killed him at the last moment, but you got my massage...).

R - SPIDER FORM / HUMAN FORM:

Elise Ulti is Spider Form / Human Form. This skill has 2 rules:
1. To let Elise switch between her human and spider form.
2. On the first point (which, by the way, you get automaticlly by level 1) your max of Spider Queen stacks is 2, and by every point you give to Spider Form / Human Form, you increse the max of Spider Queen stacks by 1.
On Elise spider from, she gain bonus magic damage to basic attacks, movment speed, armor and magic resist, and when you switch back to your human form, your spiderlings are vanished untill you switch once again to the spider form.

Early-Game:

For me, the mid game divided for 2 parts:

Part 1: The-Very-Early-Game

This part is on untill you reach level 4.
On this part, you should focus about one thing, and one thing at all: FARM! You don't attack, you don't push and you don't die. You farm, and stay alive.
If you passed this part well, you are in a good condition (which is good... because it's a GOOD condition... don't judge me...). Ofcurse there are exeptional situations, as gank of your team on the enemy midder, when you NEED to be offensive, and horribol ganks on you, which acually don't need to be on you, because you don't need to be in a situation that expose you to ganks at the first place, but if your team missed "ss" and you dead, so... good luck...

Part 2: The-Early-Game

At the moment you reached to level 4, and untill you reach level 6.
Now we had reached the fun part: HARASSMENT!
You got 2 points to your Neurotoxin / Venomous Bite or your Volatile Spiderling / Skittering Frenzy and you can harass.
If you are close to your enemy, Q him. If you don't, aim you W right and releas it.
When you see that you are in a good position, and your enemy is far enough from his turret, throw at him Cocoon, use you Human Form abilities, change to your Spider Form, attack with your Q and kill him with W and Ignite if it needed.
If you didn't got any chance to hit you combo, focus on farm. You will have to make it up by farming, which will be hard.

Now, on the early-game, there are signs which let you know if you or the Elise player in your team (or in the enemy team) is GOOD, AVERAGE or BAD:
1. GOOD Elise on this time will probebly get kill or to, will have very good farm and one of the early-game builds I have mentioned. If she is very talented, lucky or good, she will succeed to destroy her enemy mid turret, but its not likly or recommended.
2. AVERAGE Elise will maybe waste her mana too soon, BUT will be careful enough to stay alive, get nice farm and if she will be lucky, she will get kill or assist from gank.
3. NOOB Elise will probebly be killed more than 1 time, waste her mana befor leve 3, farm with skills and be ALL of her early-game on push mode and expoe to ganks. Oh, and probebly will have farm of 26, or somthing...

Mid-Game:

Also divided for 2 parts:

Part 1: The-Early-Mid-Game (TEMG):

From level 6, untill all of the teams go to the mid.
on this part, if you didn't got a kill so far, you have to get it now.
Ofcurse you need to focuse farm, and if you need to consider with yourself "kill of farm?" it 's always FARM! But if you think you are ready and control your lane good enough, go and get that kill!.
Keep harassing your opponent, but more often, and when he has 2/3 health or less AND he is not too close to his turret, start your combo, and get the kill!
Although, on this part ganks are getting more ofter as well and you should always be aware! Even if your team didn't "ss"ed, look at the map and see which champions are missing!

Part 2: The-Late-Mid-Game (TLMG):

After all of the champions go to the mid, and untill you get level 18.
Now we get serious. From now on it is begin to be very hard. Champions begin to go to the mid and the TeamFights begin.

NOTICE: TeamFights!

On TeamFights,
1. GOOD Elise should start with her human form! you throw stuns, "W"s and "Q"s to targets (not any targets! Think befor you act!), and when you find enemy champion with low health, transform to your Spider Form, jump on him with your E, bite him with your Q and if thats not enough, end with your W + Ignite and escape with Flash.
2. AVERAGE Elise will begin with her Spider Form and jump with her E on easy/squishy target, kill it and escape with much much luck or die sadly...
3. NOOB Elise will begin with her spider form, jump with her E on a squishy target which is behind all of the tanks, and get killed befor she can even say "Which witch watch which watch!"... sad...

NOTICE: Escapes!

Lets put our Elise in a sticky situation: You are in your lane, you lost your turret (sadly), you in the nutral area (the middle of the mid) and you've got the enemy's midder together with other champion chasing you, as you can see at the photo:

1 - YOU
2 - ENEMY CHAMPIONS
(lets, just for the record, that you are on the blue team - your base is in the bottom...)
What do you do?

1. GOOD Elise will change to your Spider Form, run into your jungle, use E, land fast on Ancient Golem , change back to human form, and use your W to the bush under the ancient golem to check how close does the enemy champions are, while running to the inner top lane turret and escape.
2. AVERAGE Elise will run to your base, while throw a stun to the enemy champions direction and change to her Spider Form, because of her speed bonuse and her spiderlings which can block attack like Ezreal's Mystic Shot or Rammus while he rolling right to you. This stragegy is not very good, because the enemy champions would be able easily to reach to you and kill you.
3. NOOB Elise will change to her Spider Form, use E, and get killed at the moment she returns to the ground.
There is somthing very important about Elise's Spider-Form-E while trying to escape. It is not always will save you! You need to use it only with thinking! use it to land on far minion that will get you far from the enemy champions or to netural monsters behind a well, but usually, just go up in the air won't save you!

Late Game:

After reaching level 18, untill the end of the game.
Try to live, be smart, use the tips I gave, buy items that will help you take the maximum potential of Elise on every game and good luck!

To sum all of this up:
Elise: or you are an OP champ, or it will be very hard for you to get positive score...
HAVE FUN!! :)