Both of them are dildos, so barrel them for the xp and sweet loots

I had a character wearing the pirate gear and he just kept getting charmed... so no attacks from him.

First time I fought Grog, I was testing out shackles of pain, so sorta killed Grog first round. So that's my personal tip to fight Grog.

my group unintentionally cheesed this so hard. one guy was a pyro mage, I had torturer and dipped my weapons in poison. we had a rogue who abused sleep/charm too.

As someone explained in the comments below it is possible to finish this quest but also kill both Marg and Grog for the experience.

1. Accepted quest from Grog to kill Marg
2. Talked to Marg and told him that Grog wanted him dead (received experience for telling him this)
3. Attacked and killed Marg (received experience for killing Marg)
4. Went back to Grog and separated my duo lone-wolf party leaving one on both sides of the bridge (2nd party member who will attack Grog should be left on the side where Grog will start to walk towards after finishing this quest that being Cloisterwood side.)
5. Initiated dialogue with Grog with my main character and told him that I killed Marg (Quest finished, received experience and the reward screen popping up on the character I talked to Grog with.)
6. Now you do not have much time to actually look at your rewards in the pop-up window so save/load this part if you feel like it. Pick your reward and quickly switch back to the party member on the Cloisterwood side of the bridge and attack Grog. (I started the battle with Sebille casting a fireball at him to trigger the combat and also apply burning to Grog to stop his regeneration ability.)
7. Kill Grog (received experience for killing him and also loot on his body same with Marg)

Tested on definitive edition of divinity 2 (7th of august 2021)