They just killed the mammoth, it’s a joke partially about hunting buddies, partially about the fact you can’t take a mammoth down alone, partially about him getting to keep the carcass (dismissing his friend, neither of them could move it anyways).

The hole in the side is bad drawing. It’s just supposed to indicate the creature is dead dead.

It seems like they’ve just made a business deal, rather than killing a mammoth. The giant perfectly round hole in the mammoth is a mystery though