depends. If u have gold and u can afford it , u can sometimes use irons (lvl 20 irons tho from imortal beast/base scramble2) to level units to 50 or 60 , but that is usually not that effiecient. generally 40-50 is okaish , then start using plat armors + 3 fairies of the same rarity as your unit for a whooping 8k xp per plat armor