@ LegioXXXUlpiaVictrix

AI sends their footmen forward and keep their cavalry WAAAAAAY out on the flanks to hit my flanks when I've already routed their footmen, pretty much.
I think there may be a solution to help you there. I'm guessing you are on large/huge units. You can mod a units depth so the A.I. does'nt have that long rediculous line with reserves on there flanks. Also when the A.I. has more depth to there units they are WAY harder to route. And when they do route more of the unit is preserved.

Infantry get the biggest benefit from this but you should also increase the depth of cavalry to reduce the chances of it getting hung up on multiple units.

Reccomended depths (my oppinnion), this is for huge unit sizes. 160 man infantry unit 8 ranks deep. 240 man unit 10 ranks deep. Any other specialty infantry(smaller than 160) I left at default. As for cavalry if it is melee cavalry I put them at 6 and skirmishing cvalry at 5. I made all javelins have a depth of 6 and archers and slingers 4. I got tired of the A.I. maneuvering there missile troops with ease. Also take out all horde formations minus wardogs. Paesants are still worthless out of horde formation but gives them a little more bite.