Because I can't post on the grown-ups forum...

No one has suggested the following.

The barbarians use different tactics to standard romano-greek pin and flank, namely horse-archers.

A 50:50 spilt fighting rebellious Romans with heavy infantry and horse archer should have the following effects

1. Skirmishing will tire the enemy
2. Ditto reduce numbers
3. Their lines will break

At which point (3) bring up (what will be a narrrower line of) infantry to deal with their broken lines. When engaged, your horse archers can come out of skirmish mode and be the best back-stabbers in the world.

However if the enemy has superior cavalry and know how to use it, namely agasint your horse archers, you could be shafted...

If unsure about tactics, try it out in the "sim", i.e. a custom battle.