12 gems should certainly be enough. The AI really shouldn't be using extra gems in this situation, but whateva.
I have made a couple builds w/ S1 pretenders, planning to always have them hidden amongst my mundane mages,... but they never quite make it to the finish line
Magic Duel doesn't always go after high hp targets. I had my god casting magic duel at llama's skink priests and slann in a game, and while most of the time he went after the slann, once or twice he chose to magic duel skink priests instead, even without said priests being stacked together in a group.