That depends if you want to kill it solo or with some friends. I've recently created a new Sorcerer character to get achievements and rook to make my new Monk, and since Bosstiary don't reset, I've been killing some bosses with my friends. I went to Zevelon at level 50 and even with my friends surrounding him, he hit me from distance, also paralyzing me and sending me his vampires. I don't know if a level 50 can kill him solo, it is a high risk situation because of the paralyze. Once he reaches you, you are doomed. Maybe level 80 is more safe. I would go solo with a level 100 character to make sure I succeed and also avoiding get full boxed with their summons, because they hurt a lot.
Besides Arachir, Zevelon and Diblis, there is also Sir Valorcrest in Edron.