Turns out Prima's Strategy Guide was right after all.
When combined with Uneasy, powers causing Heavy Normal Terror (such as Terrorize, Terror Incarnate, TK Storm, others described in Guide)
cause mortals to run to toilet and vomit (ReactionRunToToilet).
It seems to happen almost 100% of the time on fresh (unscared) mortals.
Using this, Raindancer can be freed fairly quicky:
1) at the start of scenario use Flower Power on mortals right away (with Stonewall or another ghost that can be bound near the entrance)
2) bind a ghost with Terrorize (like Ghastly) to the nearby axe
3) destroy wasp's nest somehow (Stonewall's Tremor)
4) use Terrorize trying to hit only 2nd and/or 3rd mortal (you don't want to hit 1st mortal - Bruce Elm, because he has very low willpower),
one of them will run to the toilet and vomit
There are probably even better ways to do this.