It's actually a matter of preference. If you want your spell to disable everything, storm bolt is the right choice then. Otherwise, I don't think there's harm in allowing units to cast spells while frozen.Then why not just base the spell off of storm bolt and make a trigger where the units in a group recieve damage each seconds?
Well, what I did was probably the simplest way of effects. I attached the Freezing Breath Missile effect on the casters chest. There are plenty more better ways to create even more beautiful effects using triggers. Check the forums for some tutorials.I got question how did you create the effect of that white circle around your caster, i'm trying to learn how to make better custom spells, i would appreciate it if you posted here (Yes i did look at triggers)