When do you intend to make the cast? Upon attacking? You might be able to use an orb effect like Orb of Lightning for on-attack casting, but Bash and Critical Strike won't help you in the least.
Event: A unit is attacked.
Condition: (Attacking Unit) Equal to (Unit-Type of <your unit>).
Action: 20% chance to cast Breath of Frost.
I don't know all the actions without looking in the editor, but it is easily do-able. You could create a dummy for this or just let the unit cast it himself. Although I don't know if the AI auto-casts such spells? If that is the case, dummy is the way to go.
Like Razed said. It's really easy to trigger. But is it an ability the unit have or do you just want the effect to happen even though the unit does not have the ability?
One major flaw. A unit Is Attacked fires BEFORE the attack is launched, i.e. you can stop your unit just before the attack launches, and the event will still fire. Thus, you can endlessly spam the Stop action and the trigger will fire as though you launched the attack anyway. Also, it doesn't wait for missiles to strike the target.
It's possible to trigger it, but if you want it to be accurate, then I highly suggest you don't, since so far nobody has done it perfectly.
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