Kog'Maw is a deadly carry with Bio-Arcane Barrage letting you auto attack from so far away PLUS deal damage as % health. I build him as an AD carry because at the moment that is how he best fits into the metagame. This build focuses on building AD, because with a lot of AD you can poke more efficiently with Living Artillery AND kill squishies in 3 or 4 hits.

- Upon dying, Kog'Maw starts a chain reaction in his body which causes him to detonate after 4 seconds (he can move in this time), dealing 100 + (25 x level) true damage to surrounding enemies. Kog'Maw will gain additional movement speed upon entering this state.

Run at people when you die, you blow up and deal true damage (cannot be mitigated). I would like to point out to keep this in mind if you think you will probably die from a gank. You can turn around and get one of your gankers low enough to kill them with your passive. By no means TRY to die, but keep it it mind.

- (Passive) Increases attack speed (Active) Kog'Maw launches a corrosive projectile which deals magic damage and reduces the target's armor and magic resistance.

When you use the active the passive does not go away, so there is no reason not to use it when you start DPS'ing someone. I usually wait to see if a level 1 fight breaks loose before I skill up, so I can take Void Ooze if I need it for a lvl 1 fight.

- (Active): For 6 seconds, Kog'Maw's attacks gain additional range and deal a percentage of the target's maximum health as magic damage (max 100 damage to monsters).

The wording here confused me when I first started playing Kog. From the wording it sounds like this effects his skills (caustic spittle, void ooze, living artillery) but in reality it only increases the range of his auto attacks. So do not use it to try to finish off people with your ult! :) This is the bread and butter of attackspeed Kog. This is what you should use every time you start dpsing in a fight, and to harass in lane.

- (Active): Kog'Maw launches a peculiar ooze which deals magic damage to all enemies it pass** through and leaves a trail for 4 seconds which slows enemies who stand on it.

Nothing much to explain here.

- Kog'Maw fires a living artillery shell at a great distance dealing damage and revealing targets for 4 seconds. (150% bonus damage to champions).

* Cost: 40 Mana (each subsequent Living Artillery in the next 6 seconds costs 40 additional mana, capping at 200 per cast)

You are the carry. You are the one that picks out the highest priority target and murders them. However, you are also very squishy yourself. When positioning for a team fight make sure to stay behind your tank and melee. Even when you're running around try not to bunch up or stand on top of your tank. Stay safely behind the front line, probably along with your support / casters.

If the teams are duking it out seeing which one should initiate first, poke a little with your R. It won't do too much damage so don't spam it and blow all your mana, but even a little damage and sight on the enemy team is very beneficial, and you have the range to pull it off.

Once the fight is initiated throw a slow in the middle and pick out your first priority target. Usually this will be the enemy carry or mage (ex: Caitlyn, Vayne, Brand, Fiddlesticks). If they are not in range of you to safely dps them with your W, switch to the 2nd top priority kill. Maybe this is a support or offtank. Prioritize Melee carries over range carries because it will be much easier and safer for you to kill a melee. If an assassin or another carry comes and tries to kill you Exhaust them and teach them a lesson.

Make sure to finish off fleeing champions with your R if you can, but remember don't chase too far and leave your team still fighting without their carry. It is very vital for you to keep dishing out sustained heavy damage during the team fight.

Some clarification on item choices:
Simply the best boots for Kog. This will net you much more damage than Sorcerer's Shoes with the AD build. If it rolls into late game and you need survivability replace zerker greaves with Mercury's Treads.

This item transitions you into being a true carry that deals massive damage. I like IE because it works well with your Phantom Dancer and crit masteries. Some people prefer Bloodthirster over IE but I will go over that in situational items.

Gives you much needed attackspeed and movespeed which work great on Kog. The crit chance will work nicely with your Infinity Edge.

This is a good survivability item for everyone, and I feel like it is quite helpful at this point in the game. Late game fights are all about surviving burst, and this well help you do it. I feel like survivability is more worthwhile than more damage at this point. You will hit a plateau where if you buy no survivability you will not last long in lategame teamfights, which in turn means you will be a non-threat.

Champions have built in armor gain through levels, and if the other team is smart they will be building armor (especially since the current meta promotes 3 AD champs on a team). Combined with the debuff from your Q this will make armor a non-issue.

Before you go "WHY MORE THAN 6 CORE ITEMS?!" listen to me. I only list this as a core item because you should replace your Wriggle's Lantern with Bloodthirster if you max out your item slots. More lifesteal, more damage. You lose the nice proc for killing baron, but the increased damage should balance it out.

Use This Build and BOOOOOOM!