So that rage ability Ardull has is, uh, interesting. First he emotes "Ardull the Watcher begins gathering his rage" alongside activating a 30 second self-buff called Gathering Rage. Looking on Lucy I see that it has a big Damage Absorb which matches my experience trying to damage him when the buff was up. He can emote a second time before it fires but the Focused Rage AoE can seemingly fire on any tick. I was watching the countdown on my beastlord, waiting for the damage absorption buff to wear off so I could hurt him again, and it definitely went off at least once before hitting zero on the timer.
Gathering Rage's in-game description reads "Absorbs damage and incoming spells" which I initially mistook for a script counting how much damage he takes while the buff is active in order to return it in the AoE but this is not the case. It's just describing the damage absorption trait of the buff. The AoE charge up can be interrupted but how to do so is something I'm still trying to work out. If interrupted Ardull will emote "Ardull the Watcher's rage has been interrupted." and instead fire off Interrupted Rage as the attack. Since the interrupted version does less than half the full version's damage this is obviously preferable. Walls might block both AoEs as the full one is instant death to my mercs even when fully healed but in several fights against Ardull I've yet to have all three actually hit and killed. Just something to keep in mind.
Ardull is also VERY resistant to magic of all types. I fought him with a Shaman, Beastlord, and Cleric team and even debuffed with Malos poison and disease spells had a hard time landing (using Malos in particular because it is unresistable). I tried stunning with the cleric and every time was resisted. The warder's procs also had a hard time landing.