Can't be mezzed, can be charmed, can be slowed. CAN BE BANISHED. Have to kill these in under 30 seconds to reset the executioner, however banishing does NOT reset the executioner. Max hit seemed to be doubles for 268/hit.
Best to kill these first each wave unless a named pops. If you get two or more of these guys in a wave, the druid and/or magician in your group can banish them for the small cost (worth it in this case) of a star ruby. This is why 60+ of those two particular classes are in such high demand for trials (that and epic pet dps).