Hi there and wellcome to my first Champion Guide. At first im sorry for my lacking english skills but this is more about the game then about the language so please enjoy.
My Main Elo is Platinum but my LoLking is linked at the bottom.

Those are the runes i run against every matchup. Well sure if your against an full AD Team then you could swap out the Magic Resist but i think thats not needed.
The Seals and Glyphs should be obvious.
The Quintessence should be obvious as well because she only has AD scaling the same counts for the AD in the Marks and the Amor Penetration is because her Spells deam Physical Damage.

So you just max Q first then W end then E and take Your R at any available levels. Not to hard:

Now we look what Items you buy. Since Season 6 or better said preseason there have been a lot of item Changes.
Starting Items:
Doran's Shield x 1
Health Potion x 1 (buiscuit because of Masteries)
(I always start this way and its a good start your early scalings are good but not so good that you needed to buy an Doran's Blade xD)
First Items:
Trinity Force and a Dead Man's Plate I rush because they give you an insane amount of damage.
Full Item build:
Trinity Force Dead Man's Plate Mercury's Treads Sterak's Gage Randium's Omen Spirit Visage
I mostly buy this Items and also in this order. If you are against an AD team then don't buy the Spirit Visage but thats obvious.
I don't realy like the Black Cleaver but its an option to the Trinity Force

So this is the part where i will explain how to play this champion.
So at first you have to realize that your passive, i mean the tentacles (i hope this term is right xD), realy imortant because they are your main damage output.
So a t first you always want those things on your lane. Do this when having your passive up by walking do a wall then a tentacle weill spawn (most of the time).
Then the earl game is not very hard you have your problems vs gnar but you will survive it.
In early levels your spells have nearly no mana cost so don't hesitate on poking the enemy with your q. Also use your w when a tentacle is near you or yous the w as a capcloser.
Your E has an realy intresting mechanic. The spell seems useless at the beginning but its very op and is realy funny. Try to hit the enemy with the e and then try to destroy the clone. BUT be carefull this is an oppurtunity for the enemy if you used al your spells on his clone. Once the Clone is destroyed the enemy is having fun xD. Well and your Ultimate is quite simple. You deal a little amount of Damage and instantly spawn tentacles depenting on how much enemys you hitted with the Ulti. This is good when you fight in an Area where you are not having Tentacles.
Your standard combos is Q W R - in teamfight use your ult when you have like 2 or 3 people around you.
Your poke combo is just hitting q and trying to to the E thing.

Pros:
- very high base dmg
- very high dmg in general even though ur an offtank
- good mobility through E
- good poke with e

cons:
- VERY affected by ganks
- has some problems in areas where no tentacles are but in most cases the ult does it
- well cc is not to good for her as well

So this was my first guide there are a lot of mistakes and it is still not very detailed but im hoping i can help some players because this champ is really funny xD
Here is my LoLking: http://www.lolking.net/summoner/euw/43117848