Has anybody gotten him as a challenger? If so, is he random? Where should I go to unlock him? Thanks!!

darling, I believe you have to visit the Ice Fields in order to unlock Hoarfrost. Once you've unlocked him, he is unlocked forever and he shows up under the Event tab in the BC. He is pretty mighty.

You've found a Hidden Opponent!

Hoarfrost challenges you to a fight in the Battle Coliseum! ahh yes!! Found him!! :D Thanks so much!! ^^

: Anytime my dear.