at 100 power he casts his debuff (Mark of the Fallen Champion - Spell - World of Warcraft)
if he targets you with the spell you have to cast vanish/feign/invis the exact same time he finishes the cast
you get no debuff and boss is at 0 power again ... with some luck on the targets he picks your group can fully avoid the enrage like mechanic on this boss for an easy kill
if you vanish too soon the boss stays at 100 energy and just picks another target
if you vanish too late you just get the debuff