Grog the Troll is a NPC in Divinity: Original Sin 2.
There is a plant thing that generates poison nearby, down the hill. Teleport that up to the bridge, place it right near Grog and set the poison cloud on fire from a distance. This will keep you from starting the fight while continuously damaging him. By time his hp is down to 400 ish, talk to him - he will die almost instantly, clearing the quest.
I beat him 5 levels below his (at level 13) and probably could have done it earlier, it's a great move for the XP and the loot...my tactics:-clean the bridge (use rain on the poison)-one character shocking him-one character hitting him with fire-other characters can do whatever as long as they aren't healing him (examine him for his strengths/weaknesses)He's weak to fire which is the only reason I thought I might beat him. If he gets a turn to act he'll heal and kill my tank in one shot, but after learning this the hard way I realized I could keep him shocked and burning every round....this makes him completely unable to heal or hit you. I didn't talk to him, I just started attacking him with a shock attack right off the bat. Later I probably could have turned him into a chicken but all it seemed to take was a bunch of shocking attacks (spells, presumably arrows would work of shocking type) and fire attacks. At one point my water was vaporized (by my fireball probably) and that created steam which made targeting him hard, luckily I had shocking touch and other close quarter methods so I kept rolling until he dropped. I'm level 16 now and still using the ring he dropped on that day when I was level 13! So load up on fire and shocking skills/spells and blast away!
If you have the polymorph surface swap skill you can use the lava near hnaag and carry it there (may take a minute)