well ceaser's legions did not fight in the gallic wars for 8 years straight to scare pompey and his legions to greece and africa :)

just keep them on the frontlines and resuply them, let the germans regroup while u put half stacks with another half stack nearby (in range to reinforce ur troops) and wait by a river crossing
BAM javelins and ur troops will steadily gain xp in a couple years