Heya and welcome to my guide about Kog'Maw, the Sniper.
If you play Kog'Maw, then you know he is great killer.
I hope you will read this guide and you will try it.

In runes, I usually use
9x Greater Mark of Desolation
9x Greater Seal of Vitality
9x Greater Glyph of Celerity
3x Greater Quintessence of Swiftness

Marks for AMPEN = bigger damage´
Seals for HP/lvl = bigger survivability
Glyphs for CDR/lvl = skills can be cast more often
Quentessences for move speed = quicklier movement around the map

In masteries, I go for 21/9/0.
Why?
Well, offensive for for bigger damage, pen and these things.
But why def? Why no Utility?
To boost a bit my resists in early game and dodge is always nice even without move speed boost from dodge. (which we DO have)

Start with boots 1 HP pot and 2 MP pots.
In 1st return get Philospoher's , then Avarice and Emblem.
Then start building Madred's.
Banshee's for survivability and MR.
At end head for Start for Aura, lifesteal and att speed for W+Madred's.

Possible items:
Frozen Mallet for HP boost and slow effect
Phantom Dancer for attack and move speed

WHY: Philosopher's Stone + Avarice Blade + Emblem of Valour ?
Solution is simple:
Philospher's because of gold generation and HP+MP regens
Avarice due to crit chance + gold
Embled because of lifesteal and hp regen (later will be build into stark's fervor)

At lvl 1, grab W.
At lvl 2, grab E for slow.
At lvl 3, grab Q.

Then the maxing order is R>W>Q>E.

The ultimate always 1st, W is your killing tool, Q for att speed passive and armor+MR reduction, E last, because you will need only 1 point in this skill for the slow.

Well, this is end of mine guide about Kog'Maw, hope you liked it and you will try it out!