It probably is the midboss harpie one, that one is super derp because the stage randoms the midboss...
Does 7-1 have a quest where there's a kill requirement for both midbosses? It could be at too.
SDGO: TW: Quantj KR: Rias NA: Quant
Elsword: NA: Edaina KR: Melodiana, Jillian
Current Interest: Puzzle and Dragons