You need 2 Archons. One needs to fly away from the tile where the Force Field is cast, and the second is needed to cast the Force Field before the Black Dragon's turn. Also, the Gargoyles are bait to lure the Black Dragon into the direct line of fire of the Titans.
My hero didn't have the Tactics skill.
Added 2 minutes later
I'm not arguing - but even without the Gargoyles, the victory is obvious with the given tactic, it would just cost more mana. I just realized that the Titans wouldn't have initially positioned themselves in the corner if all the slots weren't occupied, so you could have just taken 5 random monsters for that purpose...