Tip: Make a party with two of the required summon/characters + a fast group damage dealer and you can clear the character usage missions while also farming the required monsters.
Bahamut type x10
Golbez’ Lost Chapter: Golbez, Clad in Darkness Pt. 5 (Normal difficulty) has 1 Flawless (Bahamut type) in 3 waves, completing the mission in 10 fights. Only use this option if you missed or can’t complete the Dusky Wings co-op event, which is an easier source for Bahamut-types.
Ghost types x20
Noctis’ Lost Chapter: Royal Hope Pt. 3 (Normal difficulty) has 5 Ghosts in 3 waves, completing the mission in 4 fights.
Thanks
To the DFFOO “Monster Hunter Guide“, an initiative to list all monster locations in the game (maintained by Marvan09).
To @xcaliblur, @mzpro2, @infinitus and @Lumine who also contributed tips for these stages in our public DFFOO Telegram group, which can be found here: