Who to upgrade?

Michalis, baby! I'm working on a build with Hautclere Eff, Ignis, QR, and Fortress Def. His defense is amazingly high, and his Ignis hits like a freaking truck.

Imo, if you're lacking axe units you'll want the most all-purpose one. So I would say either titania, michalis, or camilla if you have a decent non-brave axe for her to inherit. Unless you're looking for an axe nuke in which case Freddy is busted. I have a +atk raven who rips stuff up and with LND he can definitely quad as well.

TLDR; If you want something versatile go Titania if you are worried about lancers and mages, Camilla if you have an axe that isn't a brave axe, or Michalis if you don't care about mages. If you want an axe nuker go Freddy.