I would say that the best difficulty would be H/H. Makes the campaign tough with money shortages and all, and makes you act really smart in the battles. If the game is still too easy, just give the saracens lots of money with cheats. To do so press the ~ key and then type "add_money sassanids, 40000", but without the quotation marks. Do this a couple of times and the saracens will get a ton of spamstacks fast.