Arminius what archers are you using? If your using european archers, that would be why. Except for about 4 types of them, all western archers are giant steaming piles of shit. Eastern style archers are incredibly better. In my Getai campaign, my Komatai Agrianai just destroy everything in their way, and have the highest exp of all units in my army except for my HA.

The only way to 'refill' mercenary units is to merge them with another damaged unit of the same type, you can't retrain them. *Except for the mercs that some factions can normally train/retrain, but that's a different thing.*