@ icallhimtest: The boss Ruy'Arvoz in chapter 7 you're talking about is really hard to fight if your heroes aren't equipped well enough. The life restoration spell he casts is a triggered periodical event and I think it doesn't depend on the helhounds. The best way to fight him may be striking as hard and fast as you can and try to perma-disable him. Enough damage should be able to bring him down.
I fought that boss like 10 retries.. and yet i fail.. so i skip him