He has two circular AoE's. One that is close around him that causes knockback, and a doughnut shaped one that siphons HP and MP.

The following (MSPAINT!) picture shows roughly what I'm talking about.

D is Darnus.
Red represents the AoE of his knockback move.
Yellow represents the AoE of the siphon HP/MP move.
Green represents a safe zone between these two where your mages and BRDs can stand and not get hit by either.



Note that this is not drawn to scale is only used to represent what I'm trying to say.

The "Safe Zone" moves as Darnus moves, so you have to be aware of how he's positioned.