The trigger on deep forest Arena is supposed to be for the first creature summoned.

Maybe I'm not thinking the triggers correctly, but shouldn't Crush of Wurms trigger first to summon in the copies and then the Arena activated on the original copy?

As it stands right now, you summon Crush of Wurms, the Arena triggers and then the debut ability triggers giving you 3 x 8 / 8 Wurms with ward.