For that reason, I like to use a balance of vets and replacement units. Say I bring along 3 vets, after battle I merge them into 2 full units + 1 weak unit. The weak one I send back for retraining, the 2 vets I reinforce with a green unit. That way you will gradually build up your army's experience across the board while keeping your momentum.
Besides, having short pauses in your campaign while you retrain is by no means a bad thing - it allows the enemy a chance to regroup and to build up more cannon fodder for you to kill. More experience and chances to train your troops and general. Not to mention being more fun and challenging to play.
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