Kog’Maw does not have any escape abilities, making him an easy target. Try to focus him in teamfights first.
Run away from Kog’Maw’s corpse as it will explode shortly dealing a lot of true damage.
Try to monitor Kog’Maw’s W cooldown as he loses a lot of range when it is down.
His Q is now a skillshot, making it easier to dodge. This can easily be acomplished by standing behind minions!