Fire sweep for arena defense?

Well, I'd say L!Ryoma, because he's probably the one who's gonna do more damage thanks to Bushido, provided, of course, he doubles and has some way to activate a 2 cd special(QP, probably, since Heavy Blade is not that reliable on Arena defense). Cordelia is much easier to merge, though, in case you're aiming for high score, too.

Infantry Pulsed Galeforce Heavy Blade Soleil.