Well its very simple he has 3 awsome spells and 1 is the best of them all! Gues which 1?
That's right! It's living artillery.
Its cheap has a big range and a low cooldown.
And in this build I will tell you what are the best spells masteries runes and items to do so!

Kog'Maw is a guy that needs mainly 2 things. 1 is mana and 2 is ability power.
After that he needs magic penetration and cooldown reduction to keep on pressing R.
So thats why I start with an sapphire crystal to stay longer in the game because the only reason you have to go back in the beginning is because you run OOM( that problem gets solved after tear of the goddess).
Then I take the boots for better dodge and faster movement and only upgrade them after tear of the goddess so you dont have to go back for your mana and you can keep on going with firing your ability's.
after tear you can upgrade your boots for faster ability fire.
Up next is the archangle's staff. Why? because it allows you to have almost endless mana and more ability power, that's why! If you have archangle's staff just keep on firing ability's for maximum use.
Why use evil tomb? because it increases you ability power your mana regen and most important, you have a 20% cooldown reduction which allows you to fire living artillery every half second. Amazing dont you think?
Then the void staff for maximum use of your ability's also counting for death cap.
I save them for last because my build is mostly for staying in the game with endless artillery then fire nukes in the start later I go for both.
Any improvements on the build? Plz comment

This is my first build and its no guid you'll have to find that out yourself.
I hope you enjoy it at least give it a try because kog'Maw is not made for attack, but for AP!