I attempted to split his guard with monk FD and the guards stick to him. Has 300k hps, either he never casted any of his spells or never saw them land. My group was lvl 75 WAR CLR ENC ROG MNK WIZ, they all had the Ally gear from combine with OoW era group gear. Tank had 16.8k 1055/1036 AC. I had the wiz mitigate spell dmg 25% up to 2000 dmg and never lost the buff during the fight. I did keep throat jab on him at all times.
Guards are completely mezable just like any other normal lvl 76 mob in the zone.