Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
THIS BUILD IS NOT FOR BEGINNERS.
All other guides on AP Kog'Maw just don't live up to the potential, this is mine and it's just raw.
So raw... it's a trap. Enemy heroes underestimate Kog'Maw, get to close, but find out that he can hit over 3000 damage in under 5 seconds.
Skill Sequence Explanation
You really want to focus on your void ooze, reasons being, both Kog'Maw's Caustic Spittle and Void Ooze do the same base damage when at the same level. Void Ooze is fantastic for that early game harass because it has a pretty decent range, and it's recommended you play either a bottom or top lane.
Your Bio-Arcane Barrage only comes in handy late game, when the 3% damage of the targets health (Plus added % based on AP) clause does a huge amount of damage to tanks.
Right off the start, your going to want to buy a Sapphire Crystal and build up to a Tears of the Goddess. You want that extra 200+ mana boost so you can use Void Ooze a couple more times. You should be able to stay in a lane long enough that when you teleport back you have enough money to buy out the Tears of the Goddess.
Your next item is the Mejai's Soulstealer.
Get the the Sorcerer's Shoes for that nice 20+ magic penetration. Right now, cool down reduction isn't that important in early game.
Getting the Archangel's Staff next is pivotal, the 3% of your mana converted to AP clause is great because the extra mana it caps at is 1000. A nice 30 AP added for the relatively cheap cost of the Staff.
Rabadon's Deathcap. Self explanatory.
Lich Bane is next and perhaps the most crucial to adding flexibility to your character.
When I play Kog'Maw, and by the time I get Lich Bane, I have around 550-700. By the time it gets late game, I have 900-1000 AP. Adding an extra 1000 damage due to a physical attack is HUGE.
What ever you get next is situational. If the opposing team is building magic resistance, you want to get the Void Staff (However, you won't reach 900 AP). Otherwise, just get another Rabadon's Deathcap.
If your game goes beyond that, sell your boots and get another Rabadon's Deathcap.
You see a pattern here don't you?
Basically, play conservative. Kog'Maw is extremely squishy and prone to focusing. If you play him right, and since your Living Artillery has one of the largest ranges in the game, you can do so much for your team.
Go off by yourself, especially mid-late game.
Kog'Maw feats on mana. The Living Artillery stacks up on mana cost within every 6 seconds of use. Whenever you can, get the mana buff.
Remember, Living Artillery has a delay before it strikes, aim in front of enemies for maximum accuracy.
If you keep dying and your Mejai's isn't doing anything for you, play better.
If you do it right, everything should play out like this.
Caustic Spittle - 800+ damage
Bio-Arcane Barrage - 3% (+10%) of the targets health done as magic damage
Void Ooze - 800+ damage
Living Artillery - 800+ damage
Lich Bane - (Assuming 900-1000 AP) Another, 900-1000 damage every 2 seconds.
800 is just a guestimate, it should actually be more, I just don't remember the exact numbers.
3400 damage, not including the Bio-Arcane Barrage.