I've found the boss fights quite easy if all of the characters know at least one quickening. Just have your first three chain as many quickenings together as possible, then switch out one character for one of the backup ones. Then have the fresh character use his/her quickenings, and that'll let the other two mist charge and continue chaining. Keep doing that for all of the characters until everyone's used up.
The boss'll be dead before you can say sianara.