Online I use the local outnumber bonus, flank bonus, fire bonus, scary infantry bonus, scary chariot bonus, and maybe a charge from behind bonus to pretty much win games in 2 minutes. You basically make a short thick line, hit on the enemy flank using a mass run behind(it causes your units to engage rather than stop short), push some spearmen behind the enemy to defend against cavalry as well as reinforcements and start hitting the corner with cavalry charges and fire.

That or you can sit them behind your battle line inf you're a sedentary faction.

Usually to combat this you need a massive chevroned up army of 'excellent morale' troops in a very tight and deep formation since that gives bonuses and helps reduce the outnumber bonus. But that's MP. You probably just need to keep your units close to each other to not get that insta-rout effect.