I ma trying to kill the big orc on easy with my priest and killed his minions but the guy seems immortal, healing himself regularly.

Do I need to do something special like with the undead bishop ?

there's nothing special about him. maybe try to level up a little, you'll get him eventually.

There is nothing special about him, he's just tough one. Try to use a lot of seals cuz speels are rather too slow and not always work - they can fail. Be sure to have a lot of potions of any kind with you cuz you are going to tank a lot. Running is unnecessary; it'll only spawn mobs.

Thank you

I finally blocked him on the bridge with just the two of us and spammed seals and potions as fast as I could hit the keys and for some reason this time he didn't regenerate since he was occupied beating me.
When I ran around, sometimes he would stop and heal himself.

my genius killed him :3

He casts Exhaustion, so wear some item that protects your from Exhaustion when you fight him.