[Corinthian]

Actually, the limit is 25% of hitpoints left. Not units. You could theoretically autorout without having lost a single unit. Theoretically.

However the HP limit is the amount of HP your units had at the beginning of the battle so as long as you keep summoning soulless or other chaff you can lose almost all of your forces without autorouting. You just need to replace the HP you lose.

Note, this trick dont work with units that was summoned with the turn-0 one-battle-spell command items. Like the wraith crown or bottles of living water.