The Star Crafts Zergling character plows his way through his victims. Kog'maw probably becomes one of the best carries late game simply because he has such a huge range, he can stay protected and still be able to hit the enemy which is invaluable in team fights.

Movement Quints - with these you can start off with a long sword and pots instead of boots and in end game if you sell your boots and have one or two phantom dancers you won't have to worry about being left behind.

Armor - flat armor ones give you a good chunk of armor that helps you counter ADC damage early in the game and can give you an upper hand.

MR - I'm thinking it'd be better to go with attack speed ones but I've yet to test it so I'm not exactly sure, most AP mages played now are tanky dps AP so the small MR you get from the runes won't help when these guys start doing damage, it may help counter their early game but in the end it won't really matter.

Attack Damage - self explanatory, but get the flat ones so you have more beginning damage to help put yourself ahead and it takes too long for the scaling ones to be effective during laning.

Kog'maw generally goes bottom (sometimes mid ADC works well if you have a bruiser bottom) and he starts off bad. He has no escape so most of the time if you get caught out of position go balls deep and try to do enough damage to either kill or passive kill them which will either keep you even or potentially ahead. Once you get the blade of the ruined king and his E you can "escape" or hunt down enemies really well and deal, as Phreak likes to say, " Tons of Damage ". You may want to pick up boots 1 before completing the Blade of the Ruined King but try to get it as fast as possible. It's passive with Kog'maw's W works gasmic wonders, even if the enemy isn't tanky, it MELTS their health and if you pop the active while fighting it'll be even worse the them. Pick up the infinity edge next for some more solid damage during your mid game and if you are fed you may have a phantom dancer by this point too and if you do, you're set for the game. . . anything else becomes a bonus. Kog'maw relies on these 3 / 4 items so once obtained you can't really be stopped and about this point in time you'll be doing 10ish % base of the max health each hit + your damage and it wrecks the other team. Only thing you need to do now is let your team initiate and you go on a rampage.

After his Core items go ahead and pending on the situation grab health and/or more attack damage to do more explosive damage or simply stay alive longer to wreck more havoc.

Any Criticism is welcome as long as explained. Thanks.