I have 7 Faes and 3 of them have great IVs. Should I consider a merge project?

I want to get a +10 Fae eventually. I think she's 3* now so it should be easier.

You should definitely make her a merge project. +atk/-HP is in contention for her best IVs. Fury and Dull Ranged is a good set up, especially with the DD seal, and at neutral Res, a max merge has 34, which is pretty good for the Spd Ploy.

I made her part of my arena core, and it takes a lot to kill her, but I have also given her Summoner Support and a very high end skill set.

Do it

I would definitely recommend the -HP +ATK
Granted I'm not at +10 merged just yet, but here's a preview with summoner support if you're interested.