Ah finally a real mission right? Well this one can be annoying. It's only annoying for people that hate mini-games.
Basically when you come here. Anytime between now and chapter 3. You can play this mini-game to prove to Beclem that Final Fantasy X isn't a crappy game. The tutorial is basically explained pretty well. You're generally fighting 2 things.
First the clock. You could end up with the high score only to screw up cause of time. -_-. 2nd is the enemies along with ammo. If you're aren't watching your ammo or planning and patiently waiting you'll run out of ammo quick.
The best way to build your score is to make your way towards the waterfall area. Near the end of the section there's spot that acts like a total blind spot to all the enemies. You can hang around there and kill at your leisure. The only hard part is dealing with the last enemy and keeping your chain x3 score. Cause there's usually one big enemy that pretty much is the cherry on top of your score. So save one or 2 Death/Volley bullets for it.
Once you're done here Beclem's salty. Mission Complete