Fast way to level up units?

[METHSTROKE]

Hello all, Wondering if there is any possible way to level up roman/cretan archers so that the missile damage increases. I know combat is obviously how it happens but is there a more steady or quicker way? It takes quite a while to level up my units in campaign, and as i've read elsewhere, I am playing on ultra/high unit size, so that is probably biting me.

[Jambie Lionheart]

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