Level 10 should be perfectly fine. Sarevok is a pretty straighforward fight, specially after you kill his minions.
He cures himself by dealing damage with his sword. If he he's regenerating a lot of damage and killing chars in a turn that sounds like a build issue. Your front chars aren't build with enough AC and damage reduction to stop him and/or your weak chars are taking innecesary risk and being targeted.
So, take a look at your builds and party set ups. This is one of the easiest boss fight of act III
I can't remember if you can disarm him or not, but examine him. If you can disarm him that would trivalized the fight.
Also, if I remeber correctly he isn't inmune to blind. If you don't mind a "feels like a cheat" tactic just use darkness and a character that can see in magic darkness
