Without knowing the nature of the challenge and what you want your Nina to do, it is difficult to recommend skills that your Nina might want.
For example, I want to be able to throw Nina in a crowd and just have her take care of the rest. With that in mind, I run her with Point Blank, Hoshidan Unity, Nohrian Trust, Bowfaire, and Luna while being paired up with someone equipped with support skills like Pavise, Aegis, Aether, Miracle, Dragon Fang.
I believe you mentioned that you want her to debuff enemies? In that case, something like Draconic Hex, Aegis, Pavise, Sol, [weapon]breaker might be something that you can consider. Draconic Hex debuffs the enemies while Aegis and Pavise helps her survive longer. If you just want her to only tank melee or ranged attacks, you can take off one of Aegis/Pavise for another [weapon]breaker skill. Sol is there to help her maintain her HP but given her generally low damage output, you might want to consider Renewal as that heals her more consistently. Position her within 2 tiles of Azura with Amaterasu and that is 60% HP recovery at the start of your turn. So, it all really depends on what you want your Nina to do.
You also mentioned that you have completed Revelations. If you want to grind for stats, you'd want Aptitude and maybe Nobility if you want shorter training sessions. I personally like to cap weapon ranks as I go so I wanted to get Aptitude on my units first.
Sure thing. Just let me know when you're done with a certain unit so I can put other ones up.
Thanks, just got all of them!
Updated list of desired skills: