Well theres 4 bosses already with a random element. Shiva Ex on which weapon she uses, Last boss in Stone Vigil HM, turn 10 phase 4 if theres a tether or not before the charge and turn 11, stack or spread, could say on tethers also. Forgot, t13, 1 or 2 towers on phase 4.

Having minor random elements like those above do work, basing the whole fight on random behaiviour can be problematic.