Not sure if you still need help with this, but basically you need to guide him into the negate/weakening field that Verandis creates.

As you kill adds, Verandis gains the necessary power to charge the field. Once it is active, you'll need to kite Rada into it.

Something to note, at first I had trouble with this because my pets were keep his attention outside of the field. So if you have pets, you'll probably want to dismiss then. It's not terribly hard to do without them.

While Rada is in the field he will take more damage, and before you know it after a few cycles of this he will be on his away to the Far Shores.