I'm on the part where I have to kill the hydra, but my previous experience with it is that the tank we had, 5k HP, took a hit with full HP from Triumvirate which left him with less than 900, what follows after 3-4 seconds is another Triumvirate which we could not top him off quick enough. Triumvirate itself seems like a 0.25sec cast time, it's impossible to interrupt, so how do you avoid this attack? I hear you can run circle around the boss but once the cast bar is finished (which happens quite rapidly), is it still avoidable?

Thanks.