If Frosty goes off in the first half of a round (before the enemies' turn) they're forced to pass immediately, passing instead of whatever spell they would have cast. If the enemies go first, it should force them to pass the next round.
A Stun Shield will block the stun effect, so you can't stun enemies twice in a row. Frost Giant and other stun spells leave shields behind when they stun the first time, stunning them again removes the shield.
