For an open field battle against the mongols as the english, I'd go for an army like this:

5 Retinue Longbowmen
7 Order knights
4 Armoured swordsmen
3 merc/crusader spearmen
1 generals bodyguard

Of course, this is the best quality, you'll probably have to dump it depending on the forces available. However, if you do not have any merc or crusader spearmen, use armoured swordsmen instead. Don't use the levy spearmen, they are an early unit, not suited for fighting the mongols.

Put the longbowmen in front, infantry behind and cavalry at the rear or flank. Advance, and fire at the forse archers they send at you. You outrange and outgun them, so they shouldn't be much of a problem. Then send the whole army to attack. Focus on killing their general.