-atk +def Leo

+Eff Brynhildr will make good use of that extra def, since he's going to be enemy phase. A good A slot might be fierce stance or mirror stance. Atk +3 or Triangle Adept work well as budget options. I wouldn't recommend fury because it might push him out of quick riposte range and he doesn't really benefit from the speed it gives like some other heroes. If you're running him on a cavalry team hone cavalry remedies his bane somewhat

I prefer Triangle Adept 3, TA2 is good too

As someone who runs Leo regularly, the only way to really salvage his attack is to give him a bladetome build.