Orcus does physical damage and some spell damage, both of which you should be ready for. He also carries a wand of death, although for any character with magic resistance, the main problem this poses is how to relieve him of it without him wasting too many charges.
It is desirable to get Orcus's wand of death with as many charges as possible, and to kill him before he can summon Demogorgon (perhaps along with Yeenoghu). Like any covetous monster, he warps to the upstairs to heal when damaged, which can allow him many turns to summon and to use up wand charges; ideally, you would not allow him to react to your attack at all.
He is not immune to stoning, so hitting him with a cockatrice corpse will instantly kill him. Another option is to use potions of paralysis (along with free action), and kill him before the paralysis effect ends. A mounted Knight can also easily joust Orcus with the lance repeatedly until dead; the stunning component of the joust prevents him from getting away.
If fighting him directly is your only option, you need to get onto an upstairs yourself so he can't escape. This is most easily done by using the downstairs in Orcus's room when Orcus is adjacent to you, especially since the upstairs in Orcus-town is far away. You might arrange to be in this situation by coming up these stairs, either by digging down when it becomes clear you are at Orcus-town, or because you are mapping Gehennom from the bottom.
Orcus in this form comes from Dungeons and Dragons, a representation originally inspired by a Roman god of the underworld.
He physically appears as a fat humanoid with a goat's head and legs, the horns of a ram, bat wings and a poisonous reptilian tail.
Orcus, Prince of the Undead, has a ram's head and a poison
stinger. He is most feared, though, for his powerful magic
abilities. His wand causes death to those he chooses.