Is there a way to empty teams?
Aside from feeding any of the members to another, not that I know of.
what's the best way to have the girls engage in cannibalism("practice"?) I heard you feed lvl 40 fodder to your rare guys but that's it.
For now, yes. Evolve/maxlv/maxbond your Normals, read through their stories, and feed them to your Rares. Later on in the game when you have all the rares, you may want to feed them fresh rares with the same skill (the group has subsets of Smile, Pure, and Cool "mains" that determine what active skill they get). The next logical stage would be when your team is composed entirely of Super Rares, then Ultra Rares, et cetera.
Also, is it efficient to do things on the most difficult level possilble?
Yes, especially in regular events. HD gives 46 exp (and 16 event tokens total if you don't miss any, 1 extra), and EX gives 83 exp (27 event tokens total, 2 extra). However, it is
NOT worth it to try them too early on when you don't have enough rares and can't get at least a C score, as the game will scale down your gains.
Last note: do
NOT spend loveca on LP refills until you have at least 49 max LP, enough for two EXs per loveca.
