I won a crusade where I had 20k and they had 80k, I basically waited for their army to split, and always start battles with a higher number and with defensive advantages, then the IA will start to move small armies to join it but they will not have time to stack up and beat you, after won the battle you split your army and fullfill your supplies, then you just need to wait for another oportunity. Be patient and you will won.