When you see an undead player get fiends that early, it means he's going to always coil your archmage to keep him weak. If you know this, then don't let him tank for you when you're creeping that expansion.
Overall, I think you just got outplayed, plain and simple. It didn't look to me like you made MAJOR mistakes, however he picked off so many units that when he pushed with the naga at the end there there was nothing you could do about it. I'd say next time try creeping in more obscure places because you lost a LOT due to him creep jacking you, which I think was a major influence this game.