My own approach to an enemy loaded up with gunpowder units is to flank with mounted archers (turcopoles/mxb's/ha's - whatever is available).

These guys won't do much damage, but they can cause havoc by drawing the attention of the enemy.

The damage is then done by arbalesters.

I have encountered AI armies that contain numbers of handgunners. I prefer to take them out from a distance because they are surprisingly tough if you try to mix it hand to hand.