Corrin F, Robin or Effie?

You could upgrade Robin to 5 and level Effie to 40 Effie is a tank but she can't exactly deal with the current axe wielding armored units but as long as you keep her HP% above the counter threshold you'd be able to reduce Camilla 4 hit combo to a single hit and depending on the IVs she might not even be able to make a dent...

Tho...units like Hector(s) and Sheena(s) would indeed make your life a living hell for using 2 blues you would probably need a good healer or a direct counter like Ryoma or Roys to overcome such obstacles. The current metas for arena are Takumi, Robin, Azura, Olivia, Red 5 or 4 stars Camilla and Kagero [Lemme know if I missed any Kagero and Azura seem to appear less for defense for the previous week]. You'd probably breeze through it with blues, (I got my Abel to 40 just because of the endless red Metas and due to his Atk+ growth he really cuts through them like a hot knife through butter)
But when you've reached the higher tiers/ranks you'd start to see Hectors pop up every 2 games. It would be wise to save some orbs for the future banners to prevent your setup from lagging behind.
Hectors also do spawn a whole lot for the Ninth and Tenth Stratum of training do beware of that as well.