***Coming soon***

This is not good, It's only a fun build that is annoying to fight.
Don't feed = win

***COMING SOON***

Well, you are probably wondering how this is going to work? With a 52% slow for 4 seconds at max rank of his E, it is a pretty useful slow not to mention is has a decent scaling. Nevertheless, his E is a very wide skill shot so it is easy to land. It is slow moving ability so there is always that chance you can miss the slow. It's not a big deal if you miss the slow. You can cast Q for armor and MR reduction, and if you have Rylai's there is a 20% slow over 1 second which is better than nothing.

Cast Q + E + W enhanced auto attack + thunderlord's decree. With the Q landing on a target, their armor and MR will be reduced. E does pure magic damage, W does AD + AP(scaling off a % of target's max HP).
Cast Q + E + R. This is apply thunderlord's decree, Kog'maw's ult goes more damage when then enemy is below 40%hp, which makes is great for an execute. R continues to apply Rylai's slows to make the ult easy to land and keeps the target in range.

Since you will be building AP, Kog'Maw's ult will be doing more damage than usual, this can help you poke from range and staying safe, just be careful because the mana cost increases with each cast.

Kiting and peeling, with so many slows it is easy to kite the target with an AOE slow on his E and R, you can slow the targets to give more time for your ADC or team to reposition.

Using the passive, since Kog'maw's passive does true damage scaling with level, sometimes when you know you are going to die in a fight, it is best to cast, and almost spam abilities to get target(s) to low hp and attempt to get a kill with his passive.