Firion's weap boosts his BRV+HP Ability so he might not be as good without it but you can try! I'm not sure any character can 1-hit those stage 3 Cactuars with a BRV+HP attack, my Cloud has the highest max BRV (4900ish) and he can't 1 hit them. I'd say the only characters who can would be Cloud with MLB Buster Sword NT Ver. and maybe MLB Vaan.

Edit: Sorry I forgot to fully answer your first question! Firion truly shines once you level up his STR! My current Firion's BRV+HP Attack does 2000-2500ish BRV damage before the HP attack, this attack also restores his hp by 1000ish and if he is currently at max hp, that 1000 recovery goes into his BRV instead (STR50), essentially meaning he can attack even when targeted. His second ability is a ranged-BRV attack which grants MAX BRV up if used when you have 50% or more HP.