My team right now is a 4-star lvl 32 Nino, 4s 31 Hana, 5s 32 Marth, 5s 34 Takumi, and soon a 5s Lucina I got today.
Available with training: 4s Subaki, Hawkeye, Caeda, Cain and a 3s Gwendolyn is my only unit that should be tanky. So far, from that, the highest defence is a tie between my Marth and a lvl 20 Gwendolyn. Something weird happened to my Subaki, as he has 27 atk at lvl 25 and 25 def. Still, that sapphire lance would screw him over.