Hello, this is my Kog'Maw short guide. I'm jayjaynator by the way... The introduction was short! :D

Mark and Quint
Take attack speed, attack speed, aaaaaaaand attack speed. Why so much attack speed? No magic penetration? Hell no, for the same reason why adc doesn't get sorcerer shoe. Not clear enough?
Well... there is why.

1. The magic pen does almost no difference because of the low magic resistance of the minions.
2. You have Caustic Spittle. It's a free 25 magic penetration which last for 4 second. By the way with this build 4 second is way enough to kill pretty much every non-tank champ with a well placed full combo.
3. Early on, you will rely on your base attack damage and base Bio-Arcane Barrage damage to trade. The damage of your skills aren't so great early on either. Having magic penetration on a 2% max health of the target damage is not really great for early game.


Glyph and Seal
Just go for flat armour and scaling magic resist. You are squishy as hell and those rune are pretty standard anyways.

You will need the damage reduction of the Defense tree because you won't have any sustain at all (except maybe 1 or 2 Doran's Blade).

All my masteries don't need explanation I think, except for two specific one which are GREAT in my point of view.

1. Fury because the 5% cooldown reduction is for boosting the frequency of your poke. And my Kog'maw, is not about poke at all. Make fewer stronger trade instead of a lot of weak trade.
2. Dangerous Game because during a team fight, you'r best defence is to kill them before they reach you. This encourage to do so! And actually, you pretty much lack any form of sustain, so it help a little.

Core items
So, what will make this build hurt is the max lvl of Bio-Arcane Barrage. To maximize the damage of this spell, you should max first this spell while taking ALL the attack speed steroid. In those three item, Berserker's Greaves, Nashor's Tooth and Runaan's Hurricane 140% attack speed. With your passive, rune and masteries you get an other 60%, ending with about 200% attack speed almost. That's huge. With this, you will have "some" ability power and the strongest auto attack % max health based damage of the whole game.

When you get those items, you should aim to engage in the most many team fight possible, because the more many his hit by you're on hit effect, the better it is for your damage!