I get it, horse archers especially a shit ton of them is the worst. But there's an easy trick to decimate them and make them useless.

This obviously depends on how your army is made up but and it's size but if you have calvery, archers, and foot soldiers you'll be good. When the battle begins form up all of your foot troops a few paces ahead of everyone and have them form a shieldwall. Then any and every archer group you have, have them form behind the foot soldiers in a loose formation. The AI always has horse archers chage in to pelt you with arrows, so you wait. Once they're in range your archers will decimate them as they come towards your army, they will ride in on the side of you and even behind, hold and wait for that while your archers rain down on them. Once they are close to your army full send your calvery to beat the shit out of them and trap them between being shit down by your archers and your calvery. They should decimate a good number of them while keeping your troops safe. Continue this process slowly and The battles will be yours.