Huh, spirit guardians (at least the radiant damage ones) don't damage her?
Like I don't understand what is your problem during that fight. Yes aoe spells are off the table, unless they're party/npc friendly aoe spells like spirit guardians.
With the right preparations it was super easy even on tactician. Have bloosthirst elixir up on your highest heavy hitter. Just pound on Marcus and try to get rid off him asap like the others suggested. If anything, Jaheira is more of a problem during that fight than Isobel being slightly suicidal. Like Jaheira WILL destroy her own harpers with ice stroms given enough time lol.
