My group, four heroes, all eleves, so maybe a little flimsy, keep getting pasted by hybrid sentinels. They have loads of hits, the best dice, and some really punishing special abilities. We've met them twice, and I think killed two of the six so far, both of which has come back the turn after as a reinforcement.

Are we inept at the game, or are they draconic killing machines?

They are above average I've found (just completed a Lair mini-campaign and second is halfway through). Minions don't really do that much, but the master's Fire Breath can really do wonders, especially as a one space base he can get in between the heroes (and has Fly) where a Shadow Dragon can't go. Black die for defense doesn't hurt either.