Sachys: Medival Total War 2 - guaranteed if you're having a large battle, you'll be sacrificing some units - and that's exactly what the peasants and serfs are for. I once sent an army of some 5000+ peasants against the Mongols in the hope of delaying them long enough for my crusading reinforcements to arrive.
It didnt... but the bludgeoning hell was pretty intense. My army routed with less than 100 souls left alive.
Awww...Peasants are too dangerous for the morale of the whole army! They're easily killed in many ways. They wear no armor whatsoever. Mongols are mostly archers and cavalry. Either arrows and repeated cavalry charge will easily eat peasants like snack! If the Mongols stupid enough to use melee, peasants would be put to flight anyway.
To use peasants as arrow fodders for the Mongol archers, they would be a nice target practice to increase the archers' kill count, thus their experience.
My suggestion is use massed spearmen instead. Spearmen are usually cheap and easily available. They have shield so that the Mongols archery strike would be a waste. Spearmen's weapon would be a perfect weapon to kill Mongol cavalry. Surround them with your wide line of spearmen. Move while keeping your formation intact, corner them and finish them!
Nice! :)