whats the deal with arbiter?

LA DEADEYE. killed t3 xesht (24m health) within 10 seconds but i couldnt oneshot arbiter which has 7m health. within 4 5 secs, i only able to deal 3m health (later he killed me with his weird oneshot moves) i know it has resistances, but i have %70 lightning penetration which i think should be enough. couldnt proc cast on shock effectively either. how much percent resistance this monster has, %200 or smth?