My might merc got conviction aura on him at the start of a pit run. For the rest of the game he stayed in the - resists. Normally he is maxed.. so i got him killed, got a new merc and it was fine again. So does this happen to u guys or just me lol
Merc's don't activate their auras unless they're attacking, but attacking does not guaruntee activation... so yes, it is somewhat random. And yes, they are not damage by suicide bombers (Frenzied Ice Spawns?), Stygians' death explosion, or FE bosses' death explosion -- at least from my experience.
Actually, I believe Act2 mercs activate their aura in different ways depending on what their aura is. Might mercs for example are offensive, so attacking for them would activate their aura. Holy Freeze mercs are defensive, so being attacked would make them activate their aura. Regardless, your merc needs to be in there mixing it up to active his aura.