The way the fight goes is this: After a few turns, Brass will use Alloy Blast which reduces the player party to 1HP; the fight then transitions into Phase 2 when X-ATM turns up and starts fighting with Brass Dragon. The player's job here is to try and juggle between keeping themselves and X-ATM alive while wearing down Brass Dragon. If X-ATM is destroyed, Brass will wipe the player party. You'll also want to make sure X-ATM doesn't have the Petrified or the Slow-Numb statuses for very long as Brass will start to attack your party instead.
Some things that help here is to give X-ATM Barrier + MBarrier, and to throw some healing at it once in a while. If it's HP falls too low, it'll give a warning message and you'll want to get it healed immediately. As for your party, assuming that X-ATM isn't under Slow-Numb or Petrify then Brass will only use Zinc Wing (physical attack) or Alloy Ray (inflicts Petrify, single-target) on them.
Now for Brass Force; all Dragons in NT have a counter-attack that's triggered whenever they're struck by a Limit Break. If you avoid using Limit Breaks on Brass then he shouldn't use Brass Force at all.
