Had to go up to 10 units to get the 3 star with only commons here. It's not hard killing the first 50ish enemies, but the last dozen are where the stage breaks you. It's enjoyable doing this sort of thing once in a while.

>>1703240
Farming more than a few is a matter of convenience more than anything. I personally have 20 on a bunch of units just so I don't have to bother equipping it when making squads. Since this game does squad loadouts though, you only need 2-4 realistically. 12 if you run an all permaskill meme squad. They're still one of the best skill DPS subskills in general though. It really depends on how many gold keys you have to spare really.