How to Complete A Dashing Duel
The objective of this battle is to defeat Al-Cid. The law forbids harming Viera, so if you don't want to break the law, you can't harm any of the other enemy units, because they are all Viera. You should disable your Counter reaction abilities to avoid inadvertently hurting them, and be careful of getting Charmed, Berserked, or Confused. You can use a Knot of Rust to fix a unit that has been Charmed, Berserked, or Confused.
Alternatively, you could deliberately attack the Viera units, because you will get more loot that way, even though you will miss out on the law bonus. It's your choice.
Al-Cid's weapon is a gun.