I saw a post that said using Auto-calc tends to make it happen quickly. Can't vouch for that, tho.
The key is not just killing a lot of the other guy, but, having YOUR guys survive it. You need to put a good unit against a poor one or a disadvantaged one. Say, at a bridge defense or a flank/rear attack that annihilates a unit.
Too often a unit that kills a lot of foe also takes a lot of losses and the potential SwordMaster gets killed, too.

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Wind fells blossoms, rain
fells steel,yet bamboo bends and drinks