oh.. um... if you want more than one caster, you'll have to duplicate the trigger and the variables for the other casters.... I've never tried thought... eeeek... might screw up those "Event Response - Last Created Unit" lines.... one way to possibly prevent massive lag and/or conflicts with multiple Frozen Fields is to slow down the Periodic Event for the Frost Novas, and maybe extend the duration of the spell itself to compensate.
I had intended this spell for any scenarios or cinematics.... not melee maps

this spell is currently too overpowered for melee maps