I'm currently on my fourth and final run, now at Chapter 16 of the Blue Lions. This time, I've recruited every available unit, and am keeping them all levelled/used equally, so that I can pair them off as I like at the end of the game.
Petra, Dorothea and Lysithea are all ridiculously good, I've used all in every run so far, and they're so useful I can't get them off my team. Petra dodges, doubles and crits everything so she's invaluable, Dorothea can use Meteor to hit the boss on the first turn, and Lysithea is just a walking nuke, nothing stands in her way.