|The skill refinement on Leo's unique weapon allows him to fill a fairly unique niche as a sort of magical, ranged, cavalry Hector: Marquess of Ostia-alike. The goal with this build is to maximize the utility of his unique skills to prevent enemy follow-ups while guaranteeing his own in as many instances as possible. It also aims to make him relatively self-sufficient, as opposed to relying on team synergy. (That said, combine with an Ally Support from Corrin: Fateful Prince using '"`UNIQ--item-390--QINU`"', '"`UNIQ--item-393--QINU`"', and '"`UNIQ--item-396--QINU`"' to maximize potential.)
High defensive stats in combat plus '"`UNIQ--item-399--QINU`"', which will activate in any instance when the effects of both '"`UNIQ--item-403--QINU`"' and '"`UNIQ--item-406--QINU`"' apply, ideally will keep his HP above the threshold for the latter. Use his native '"`UNIQ--item-409--QINU`"' instead if your team has a viable healer to allow him to follow-up attack (or at least do equivalent damage) in the player phase sometimes when needed. One team mate using '"`UNIQ--item-412--QINU`"' or '"`UNIQ--item-415--QINU`"' should be enough to sustain in this case.
I know that stacking '"`UNIQ--item-418--QINU`"' as both Passive A and Sacred Seal tends to be the go-to for enemy phase-centric builds like this, but... poor Leo needs the Attack. He can't afford to lose the defensive stats for '"`UNIQ--item-421--QINU`"' nor the HP for '"`UNIQ--item-424--QINU`"'. If you don't want to sacrifice any of the exceedingly rare units required, then a flat boost to any stat (except Speed) is still nice. '"`UNIQ--item-427--QINU`"' is also chosen for the sake of raw damage, but it's not irreplaceable if your team needs a different utility.