I have Est and I did a bit of grinding with her since she started out so shitty (is this just a thing with pegasus knights?) so the swamp missions went okay. The main strategy I ended up resorting to for those was placing Genny in the swamp since she's unaffected by the terrain damage and making the units I needed on the other side pass her so theyd lose the 5HP from the swamp and gain them back via Genny's Soothing Light.
It was a bit of a hassle but I managed lol
So BASICALLY im in act 4, Rigel Castle, and earlier I randomly started wondering "huh, i never DID get palla and catria in Celica's route" so I googled and realized I completely missed them so tl;dr im just a complete moron