Idunno, that depends on how many actually useful fighters you have.
My personal strategy
Bronzes: level up to 30, compress 4 lvl30 bronzes into a silver. Because getting up a bronze to 30 is easy.
Silvers: level up a bunch of silvers to where you can't level up them in a couple of fights anymore. Feed them to a lvl1 silver (this way you get more skillpoints than you'd have if you fed low levels to high levels), then get one silver to 40 and evolve it with whatever leftovers you have starting from higher levels (again, higher level - less potential skillpoints - less useful).
Golds: keep duplicates for potential evolution, level them up slightly for some easy skillpoints, again until leveling them becomes a pain.
So most action happens at silvers, and it involves some healthy cannibalism (feeding highs to lows, not vice versa, except when you want to get a silver to 40 fast and evolve it ASAP).