Let's give Koggles %effective health damage
This would behave similarly to true damage, but it would scale up in damage if Kog got armor (or magic?) penetration. But at the same time, % damage reductions would still be able to reduce its damage. The only catch is it would also scale up with the enemy's resists.
The idea is to make this his passive, and make it so he heals for half of the damage dealt so that he is at least somewhat viable.