If you are fielding a Camillan or Polybian legion, deploy your army as usall, then when the enemy is close enough attack with levies/Velites supported by Hastati and Principe in losse formation. Slingers will support your attack from a distance. Use cavalry to attack already fighting enemy units, running after horse archers is a waist of them. Pray Jupiter Maximus and have faith in your tactic.

Fighting with a Marian or Imperial legion is a lot more easy. If your opponent is not using anlimited arrows, just sit down in Testudo formation and wait until they exhaust their ammo. Using the above tactic works fine too.

Using this tactics their is a 50/50 chance of winning with heavy casualties.

Cheers.