Kog'Maw: Attack Speed/ HP% dmg

I use runes that i currently have and use on pretty much every champion:
The reason for magic pen marks is that your W's % of health that it damages is reduced with magic resistance. Thats why you get sorc boots and the Mag Pen Marks for 28.5 magic pen.
You need hp on every champ thats why i use hp at 18 seals.
AP at 18 glyphs because AP increases the % of hp per hit you do with your W and increases all your other moves dmg (although not what this build focuses on, it is nice to have that additional dmg output).

The masteries i have chosen are just my preferance.
I would either go 21/9/0 or 21/0/9.
Depending on if you want to be a little less squishy or not.

Exhaust is a must! You get a nice slow so you can keep hitting with your auto attack while W is active, it lowers mag res and armor, and its good for running away if you need to.

Flash or Ghost is the next must have. Its up to you. I like to flash out and surprise attack or flash over walls to dip. Ghost is good for chasing, running (if you dont get cc'd), and getting to lanes to protect towers on time.

you can get early champion kills by leveling W first, not only because of the HP % of dmg it does, the range increase is awesome.
you only want one level in Q for the start because it isnt that helpful til end game anyways.
Your E's slow is much more important then Q for chasing with W, or slowing down champs to help you and your team run away.

Obviously get your ult when you can.

This is not a guide, im assuming you know the general way to play this game when you read this.
gank smart, lane smart, get fed. pretty much the golden rule to LoL