It does bear noting that the maps mentioned in the FAQ as the best for farming are very out of date, as more recent maps have come out that are much better. That said, you're still a ways away from reaching those maps, so needn't worry about it for the moment (It will be something to look into once you reach the desert maps, and while the simple answer it Base Scramble II or before you get there Fiery Oasis, when attempting to level specific classes of units, it can vary between a handful of maps, the specifics of which are best I don't recall off hand).

Also on the topic of using spirits to level units, while in general you'll want to match spirit rarity to unit rarity, after a point you'll not really be using iron or bronze units, and as such you might as well use their spirits in combination for any higher rarity as a minor xp boost.