Okie... I'm not too good with triggers specifically, but I have a theory for how to do your Frozen Field spell....
Make two custom units, one as the Frost Nova caster and the other as the Frost Nova's target. Both units should either be invisible or ghosted or have no model (I'm guessing the last option is best). Base your skill off something like Starfall. Then, using triggers, have it so that during your spell, the trigger creates your custom units (always in groups of two, one of each) in random spots around your Hero (I do believe you can set the offset range...) and issue orders for the Frost Nova caster units to cast Frost Nova on their corresponding targets.
Whew..... hope I wasn't too confusing.... I'll try to make that if I have the time. It seems like a pretty good spell. GL with it. 8)