Yeah, Orthus is anything but overpowered. In literally every single game I play, if he spawns far away from me, 5 turns later I get the message "Orthus has been killed". If the AI can cope that well, it basically means that Orthus is a non-entity. And this was when I was playing on Noble as well, so the AI was getting no production bonuses, and I modded out the AI vs barbarian bonus. He's literally a ton of free xp and a shiny axe.
I suspect the problem is that you're not building enough defenders, and not building them early enough. As is often repeated on these forums, in FFH you really do need to have more defenders than you would in vanilla BTS. And you need to be building them before Orthus even spawns, not after you've spotted that he's near. In one game I had him spawn literally right on my border, only 4 moves away from my capital. Not even enough time to chop out a single warrior (I was playing on marathon).
SurahAhriman is right when he says that you absolutely cannot afford to let Orthus accrue any xp though. You say that he's destroying your warriors faster than you can replace them. My question is, when he gets adjacent to your units, why is he surviving for more than one turn?
You really do need to have adequate defences so that when he rolls up at your city, his first attack is also his last. Don't let him keep attacking, because he'll just keep killing you, and if he's got enough xp for the march promotion, you won't even be doing any damage. And even if he doesn't have march, the extra xp he gets from killing your warriors will be enough to get him a promotion and healing. Instead, let him get off one attack at most (to benefit from the warrior's city defence bonus) and then hit back with the stack that you've been keeping in reserve. You have been keeping a stack in reserve, right? This can be anywhere between three warriors and lots, depending on how many promotions Orthus has managed to pick up so far. If you need to, you can even temporarily draw some defenders away from your other cities (but don't leave them totally undefended), just make sure that you have enough warriors to take him down in a single turn.
Edit:
I hadn't heard that name before. I like it. It's very appropriate. Poor Orthus. He just wants some friends to play with his shiny axe (the sharp end). Instead they steal his axe and make him cry.