Regice can be caught in Iceberg Ruins which is inside Mt.Coronet. However, it is only accessible if you have the 11th movie Event Regigigas
The extra cave is located inside Mt. Coronet, near the exit to Route
216. A "statue" of Regice is present. By stepping on the 7-dot pattern on the floor and walking up to the statue, Regice will attack. It is
at level 30. It can be rebattled multiple times as long as the Event
Regigigas is in the party; however, it can only be caught once.