Kog'maw can shot from a rly far (save) location and do alot of dmg and get kill's at the same time dosent take any dmg bc of his ulti it uses 40mana per cast 1sec or so cd but trust me ur mana woud go out fast that's why i use some mana rune's and items for mana regen and flat....
my other rune's are simple some magic pen. flat ap and cd red. for ulti u can get it like 0.5 sec a cast...

1.st Ghost i use it bc kog'maw can't exscape and has no excapeing ability
2.nd Flash this can get u out of some situation's handy if u know how to use it u can combine it with ghost for ultimate escape metod but remeber good defece is bether offense u can chase with ghost just speedeing and throwing ulti around and flash if u are missing some range for a kill... but u can use some other ability's like exhaust it can slow enemy while u shut them with ulti and slow them but i woud reccomend flash,ghost....

i go with 9 points in offense and 21 utiliy prity standard but i take inproved ghost and flash

stay back do alot of dmg by carefull some one not gank u and ur good one tip u can shot from bush so they dont see ya ;)