Eir might be one of the best F2P unit

Especially when it comes to chain battles, with her B and C skill's you don't even need to run a heal sure it will take a few turns to heal up everyone and she needs to attack just too heal 2 hp but still rather nice since she can sit in a corner and heal the unit with the most hp lost 10 hp every turn.

With someone like Arvis and his B skill he heals 20 hp every turn, and throw Titania in the mix for a triple tactics with Spd/Def/Res Tactics the team could be rather nasty

Possible build for Arvis which I might work on getting now (buffs are from Titania obv) and C Skill was kinda a mess up but not sure what would be a good skill for that slot for him

-Blue Flame just their for score obv Iceberg could work just as well