You probably waited a bit too long to take out Orthus, so he's likely got Combat I-V and Shock I-II, for +100% strength and +80% against melee. That means that he's effectively a strength 15 unit (I think, somewhere around there) compared to your strength 3 or 4 warriors. Not gonna work.
Use catapults to bring the city defense to 0, and then to weaken Orthus as much as you can before using a few Shock or City Raider Pyre Zombies to take him out. Keep in mind that Catapults can only weaken, but also have 80% withdraw rate, so you won't lose too many of them. Promote them down the Drill line if you can, since neither Collateral or Bombard damage increases will help against a single strong unit (Orthus).
Horsemen might be useful too if you've got them, since they won't be taking the extra +80% vs melee.
Luring him out with a worker probably won't work, since the barbs will leave at least 1 or 2 units to defend the city, and more likely than not will send a warrior to grab your worker instead of Orthus. Using something strong enough to defend against a barb warrior might work though.
Oh, and whatever you do, don't attack piecemeal. Letting Orthus get even more XP will just make him tougher.