By "Cure all", I'm assuming you mean an MT (multi-target) Cure spell. If that is the case, then no, no Relic is needed; all you need to do is press L after selecting the spell (both out and in battle).
God is just someone who is playing the Sims and is so bored that just kinda letting them **** up is what he does for fun, but sadly we're the Sims. - king gimpy
Unlike Previous games, and similar to later games, there's no distinction in effectiveness based on # of targets. There's the ST version and the MT version, that's that. MTing a spell is 1/2 damage (with some exceptions like Meteor, which ignores this) of a single target spell, regardless if there's 2 or 6 enemies. This is contrast to games like FF4 where it'd actually split damage based on # of targets, so 3 targets being hit with the same spell would take 1/3rd damage, while 2 would take 1/2.
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The reason that you have to press L or R to target multiple targets is this game added the Side Attack and Surrounded (Pincer Attack in SNES) formations. Clearly you can't go all the way to the left to target all enemies in one of those two formations.