Krug the Troll is a NPC in Divinity: Original Sin 2.
He heals for full HP every turn.
I'm pretty sure that unlike Grog and Marg, there is no way to remove the troll blood buff, so you just need to do more than a third of his health a turn. I had my scoundrel/polymorph kill him by preparing on one turn to burst on the next. On the first turn i used venomous coating and got into position, then had a different character use clear minded, firebrand, and haste. On the next turn I used Apotheosis > Flesh Sacrifice > Daggers Drawn > Adrenaline > Chameleon Skin > Mortal Blow. That killed him when he was at full health.