Best way to use Rosetta as a leader is when you can take advantage of extremely high hit count attacks of multiple units to generate a lot of burst crystals. This doesn't quite come into play until you 6* more units and unlock their SBB. For example, with two 6* Lario using their 40 hit SBB, even if you only manage to spark half of the 40 hit SBB (20 sparks per Lario), each Lario will get on average 30 BC back, PLUS whatever BC is generated from hit drop checks. You pretty much guarantee almost if not full time SBB spamming. It's hard to get that benefit when using only BB attacks that have 7-18hits; you'll only generate 4.5-13.5 BC per unit that sparks.
Edit: Dark unit rate ups for RS on Sunday. Think it's worth it to attempt to pull for a Shida?
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