Guard Scorpion
While Guard Scorpion has some pretty strong attacks, Cloud and Barret should be able to over take it easily. Start by having Cloud attacking the boss with Bolt attacks and let Barret attack it with his arm cannon. Do not let your party's HP fall under 100. If they get that low immediately use some potions. After a bit the boss will start to shake violently and it will raise its tail. At this point stop attacking it unless you want to get fried by Guard Scorpion's Tail Laser attack, which does 75 damage.
Once the boss puts its tail back down you can safely start attacking it again. If you are ale to gaina full limit break bar use them. It is just a matter of time before you will beat the boss, keeping your HP high and not attacking when the tail is up are key stratagies to make this fight as easy and as fast as possible.
Guard Scorpion drops an Assault Gun when killed.