King of the mists not affected by thunderstorm

Currently fighting the king in the mists and he cannot be drenched or targeted by any of the lightning strikes from thunderstorm. It looks like they are targeting him but he is not being shocked or drenched when clearly in the radius of the storm.

(Edit: thunderstorm was not at high enough level)

Just to be sure, is your Thunderstorm gem high enough level? It can only affect mobs up to a certain monster level.