He didn't weave anything but Fireball. I tinkered with the flags a bit, but couldn't get him to opt for Flame Strike, which would've been more acceptable.Finally opted for NO-FIRE. So now the mob's weaves fizzle out, but Dreadlords and LS channies will also have to work around Fire based weaves, which honestly is a fair compromise. Gain some, lose some on both sides. The mob isn't terribly high level, so I added two Elite Ghars.
This solution saves me moving around mobol on the mob, and leaves a target/ usable mob for quests, which, yes, is a consideration for the Upstairs, along with Other Considerations.