Gallagher does 95%-100% of what Lingsha does for 24k fewer jades. She is never the most efficient vertical investment for an account, since Firefly is horrible at 0-cycling any 2-hp bar boss other than her dedicated shill enemy unless she has e2 (read: the cheapest firefly 0-cycles of actually difficult enemies will always include her e2).
00:00 - 2-cost Firefly
02:39 - 3-cost Feixiao
06:03 - Builds