This video is sponsored by the Level 1 Cleric spell, Sanctuary.
TLDR:
Use Sanctuary on Isobel before the fight starts ( Shadowheart has it ). This forces the enemies to attack someone else, which gives you time to kill Marcus before Isobel goes down. It becomes a fairly easy encounter with preparations ( I haste Lae'zel and Karlach pre-fight but it's not needed ).
Long answer:
After spending 1 hour in the aftermath of NOT saving the inn/Isobel ( Halsin dead, smith guy dead, pretty much everyone dead ) I thought to myself: " There's no way this is the way it's meant to go '' so I did what any responsible RPG gamer would do and save scummed, rolling time back 1 hour.
The encounter seems impossible on first try ( and if the bad guys swarm Isobel they can kill her in 1 round ) and it's clear Larian wanted most players to fail here, but it's actually incredibly easy if you can keep Isobel alive for 1 round. Marcus isn't anything special and his flying monkeys are only dangerous because Isobel is a 50 hp NPC with an AC of ''please hit me''.
The only thing that can wreck things is if Isobel gets a good Initiative, at which point she yells "LEEEEROY", starts attacking and gets Sanctuary dispelled. Other than that, it's a Bonus Action spell with no Concentration so Marcus gets clapped.