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.
Nashorn x25
Story Act 3 Ch. 4, Pt.1: Quick Encounter Pt. 1 (Normal difficulty) has 2 Nashorns in 2 waves, completing the mission in 13 fights.
Shinryu Types x10
Beyong the Ivory Scales (Co-op) has 1 Shinryu per battle, completing the mission in 10 fights.
Thanks
To the DFFOO “Monster Hunter Guide“, an initiative to list all monster locations in the game (maintained by Marvan09).
To our friends who also contributed tips for these stages in our public DFFOO Telegram group, which can be found here: https://t.me/DissidiaFinalFantasy