Forgot to put the madred's in the item build. Magic penetration is great for kog for a few reasons. For one, all of his abilities do Magic Damage. Arcane Barrage does % magic damage based on their health. Madred's even does % magic damage. And as far as mana issues go, I find that being frugal and using the skills wisely, harassing with barrage(50 mana) instead of throwing out 110 mana slows, I can do well and go back for mana when I need it. Maybe grab a couple of pots if you're doing some serious lane work early on. Late game, mana shouldn't be an issue unless it's a real grind match.

Frozen hammer helps with survivability, gives 20 attack, and slows. If you can't see the benefit of survivability 700 hp, and a slow on hit, I don't think you understand how to maximize damage.

It is a commonly accepted fact that kog maw should buy a madred's bloodrazer. It synergizes with his w and increase his attack speed/damage output. Frozen hammer is completely unnecessary as you should utilize your insane range, plus ghost and flash to keep out of harm's way. Your team should realize you are carrying and give you lizard buff whenever it is up so the slow is also unnecessary. Your boot choice is also questionable.
Additionally magic penetration is not a very good choice for kogmaw. You have absolutely no utility to that mastery set and won't be able to address mana issues which are important for kogmaw.
People come to this site for legitimate advice please don't post stuff like this here.

haha i like the skill sequence. so much win. +1