All the Colosseum bosses are easily beatable at level 65 characters. Category 6 bosses are completely doable. All but one. Those rabid dogs are impossible. Category 6 can't be beaten with any team on level 65. I thought Vader would give that chance, but no, the dogs will wipe out the entire group at a certain point, no matter how equal their HP.
The Colosseum is already a terrible game mod. But You even managed to make it even worse with one of the bosses impassable in free-to-play.