One tactic that you can use with a fast army (cavalry and fast foot) and an enemy army that is not so fast is to get them to chase your light/missile cavalry around for a while, and then hit them when they are tired with a fresh(er) unit. The combat ability of an exhausted unit drops dramatically. This would probably require that you turn off the time limit for battles. This tactic works even better in the desert, where heavy Catholic armies can fatigue very rapidly.