build Leo +def/-spd

While CC and Res boon are optimal, I don't see why you can't just run him like he's usually done. Give him Raourblade, slap him in a Cavalry team, let him get to work. For his A as a (temporary?) substitute, I'd say give him HP+5, Res+3 or Fury as a means to boost his bulk, depending on what you most need coverage for within the team.

Either QR3 fodder or slap a bladetome and stick him in a horse team. I have him with a +atk and he still struggles to function well without a horse team unless the other side is mostly green