Re: Harad is too strong!!!
Arrived too late
anyway, in general when you have to fight against overwhelming factions, the best thing you can do (apart from using blitzing strategies as seanbg properly suggested, that is ALWAYS useful) is to approach them carefully. Study their allies and enemies, try to hit the first ones and give help to the seconds to open up other fronts for them and split their forces as much as possible and when ready storm them; use hit and run strategies as much as possible (raiding/looting avoiding cities with garrison script) use small cavalry army to intercept and vanquish their wandering small armies and keep the war on their land. Use of some assassins or tring to surrond and kill their best generals if they wander alone on the map would be useful too.
At least this is what I would have tried, but probably in your situation it wouldn't work. A pity, after such an epic campaign ..
Flinn