Krug the Troll is a NPC in Divinity: Original Sin 2.
Even without Terrain Transmutation, this troll can be killed by simply teleporting it to the lava below. It does take some fidling to be able to get your caster into a spot where he can "see" the lava below, but this is still overall an easy fight with the teleport spell.
The easiest way to kill the troll, just use the terrain transmutation spell on lava nearby and kill the troll with it
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.