Hey! I'm BoundlessGamer, a Platinum 2 player on the EUW server. AP Kog'Maw has recently turned into one of my favourite mid lane picks. This is my way of playing him. I hope you enjoy ^^
Also, this is my first guide, so please leave any suggestion/comments you may have!

AP Kog'Maw relies on his long range to poke down his enemies from afar. Think of him as Nidalee 2.0 XD

Pros:

High damage;
Huge range;
Good facechecking;


Cons:

Low mobility (low CC and 0 escapes);
Weak early game;
Squishy.

When you know you're going to die, fight back, don't run! This passive might get you a return kill. Never plan on dying to use this though.
Decent armor/mr shred that can turn a fight around. Max it second, after . Cooldown/mana don't go down with points in the ability.
Gives you some good extra damage on your auto attacks and some attack range. Useful for AA poke and farming. Take 1 point in it at level 1 and leave it there. This skill is core for AD Kog'Maw, not AP.
Your main damage source along with your ult. Has a good AP ratio and a long range. You can easily poke the enemy laner with it, and once you get some AP, a plus a on the melee minions will kill an entire wave. Mana cost goes up with points in the ability.
The core of AP Kog'Maw. It's huge range allows Kog'Maw to become a poking machine. It also briefly reveals the area it lands on, making it for facechecking bushes. It's base damage and it's AP ratio may not be the best, but it's long range and low cooldown makmakee up for it. With a full build you will have a 1800 range missile on a 1 sec cooldown, hitting enemies for around 400 damage (plus ). It's mana cost goes up by 40 with each subsquent cast, so be careful!

On AP Kog, you normally take and , since Kog'Maw is a squishy champion with no escapes, so these summoner spells will help you a lot (that's why the Distortion Enchantment is so good in him).

Other viable Summoner Spells:

Good versus enemies who can burst Kog down in seconds, like Le'Blanc, Syndra or Veigar
You can take this versus assasins who have an easy time gapclosing in on you, like Zed, Fizz or Talon. Helps a lot!

NOTE: Barrier/Exhaust are personal preference. You can take Flash/Ghost versus any matchup and do well, as long as you abuse Kog's range and don't get too close. If the enemy can't get to you, you don't need Barrier/Exhaust :)

Core Items:

Kog'Maw needs a lot of mana to keep ulting and poking his enemies, so this item is a must have.
Gives you a lot damage and some movement speed, which is nice, given Kog's lack of mobility.
Increases your AP by a good amount. Turns into once you fully stack your Tear, giving you a nice shield, which can save you.
Gives you some HP, a lot of AP and makes your abilities slow enemies, making it that much easier to hit your ults :)

Offensive choices:

Pick this up if you want an immense boost in damage. It's an extremely good item on any AP caster, and AP Kog's no exception.
Enemy is stacking MR? Get this!
The other magic pen option for AP Kog. Never tried it, but the HP, magic pen and passive hp% dmg, which is doubled due to Rylai's slow makes it a good purchase on Kog in my opinion. Try it if you want.
If the enemy isn't building much MR then this item is a good option. Gives you a good amount of AP and some mana regen, as well as some CDR.

Defensive choices:
Since Kog is a squishy champion with 0 mobility, it's often a good idea to get a defensive item to help keep you alive

Good versus poke teams, due to the spell shield and the hp regen, or if the enemy has a hard engage, like Malphite's or Vi's , for example.
Good versus fed ADC's or AD Assassins.

Not recommended items:

Given Kog's huge range, there aren't many things that can get to you. But for those that do, Flash/Ghost/Banshee's/Randuin's usually take care of the problem, so there's no need to buy this.
Takes too long to fully stack, doesn't give as much AP/HP as Rylai's does and you get enough mana from your Tear anyways.

Early Game (Laning Phase):

Pre 6 just farm and poke your enemy with some W autoattacks and some long-range . Once you hit level 6, it's time to get more aggressive, as you outrange most mid laners by this point. Poke them hard with your and force them out of lane, or even kill them.

Mid Game (once a turret falls and roams start happening):

Group with your team and take objectives (drake/turrets). Poke the enemy team down to force them to back off, so your team secures the objectives. Once you hit 11 your poke is immense, so use this to your team's advantadge. Don't get caught or your team will most likely lose a turret/drake.

Late Game (when everyone is finishing their builds and both team stay grouped, looking for picks that can give them the game, as the death timers here are extremely long):

Group with your team, and look for picks. Once you hit 16 you outrange almost every champion in the game (expect for global/semi-globlal ults,like TF's or ziggs's obviously). 1 death can give a team enough time to end the game, as players take around 60/70 seconds to respawn, so never travel alone. ALWAYS use your / to facecheck bushes.

Warding is extremely important, especially for Kog'Maw, since he's a squishy champion with 0 mobility. Therefore warding is essencial to protect you versus ganks and roams.
Green dots represent good spots for your
Pink dots represent good spots for your
Wards are ALWAYS a good purchase. More wards = more team vision = more options for your team :)]

This is the end of my AP Kog'Maw guide. I hope you have as much fun reading it as i did writing it :) I'll probably add more chapters/images in the future, but the essencial information os here. This is my first guide, so please leave any suggestions/comments you may have :) Tell me if you liked it (or not) and if there is anything you would like to see added into the guide. Enjoy!

-BoundlessGamer