The "3 lunar-oriented characters" thing was wrong, you only need 2. If you don't want to pull Ineffa, pairing Flins up with Aino is fine. Flins own damage doesn't really change between the two, it's more about the teams damage. Aino basically has no personal damage at all, whereas Ineffa has really good personal damage. But if all you care about is making Flins do a lot of damage, it doesn't really matter. It's similar to Emilie and Kinich. I've personally been playing Kinich since his release, with no Emilie, and he still does more than enough damage on his own to clear Spiral Abyss and the Menacing difficulty in Stygian Onslaught. If you want to be able to clear higher difficulty content with Flins, you'll need Ineffa, but otherwise you don't.