for kog'maw just follow this build no questions and basic attack.
1.) dorans blade
2.) berserker's greaves
3.) vampiric scepter
4.) phantom dancer
5.) bloodthirster
6.) sell dorans
7.) zeal
8.) infinity edge
9.) phantom dancer
10.) bloodthirster

you can also change one of the phantom dancers for weaker basic attacks but u would get the nice sheen passive when spamming ulti and a 25% chance of slow which could really save you if you have to kill someone before they reach you