SaGa Emerald Beyond (Sigunas): VS Observer

Step 1: Get the Speculation Formation. If used properly, Speculation essentially lets you start every fight with what is essentially 8 BP after you factor in all of the BP reduction costs that comes with that formation.
Step 2: Put someone who has Airwave or another high power AoE skill (though Airwave is generally the easiest to use, if you're counting on killing things before they act) in position 3 so they get -2 BP cost on all of their techs. Positions 2 and 4 also get -1 BP cost, preferably someone with a chain skill as most chain skills have a base 2 BP cost.
Step 3: Laugh as you get to spam 1 BP cost Airwaves from the very first turn onwards.

[@nykita427]

How do you trigger this fight?

[@Alilatias]

As far as I'm aware, you just need to complete all events in this world (and maybe you have to resolve all events in the best possible ways too). I think you have to be playing as Sigunas to trigger the Brutal version of the fight (my Diva run only ended up with the Hard version).