Anyone standing in the affected area, make a DC 17 Balance check or fall prone.
Latoris takes 5 + 7 fire, 1 + 10 fire, and 3 + 8 fire from xbow bolts.
Veil takes 4 + 4 fire from a rock.
Gogo takes 3 + 9 fire from a rock.
Breaker takes 3 + 6 fire and 2 + 5 fire from two rocks.
Quigo takes 1 + 5 fire from a rock.
I haven't included damage reduction, though I did factor in miss chances.
Anyone who wanted to add a "defenses" line to their charsheet post, that would make my job hella easier.