Just above the line, deploy some Defenders and perhaps a few Shooters as well. Place the multiple ones on top of each other this way, if the enemy is destroying the lower one, it's getting hit on the head by the one above. If money is no object, set up two tristoners on the left path (few units try to go this way usually and these are easily rebuilt), two on the very far right, and three tristoners on the middle. Note that you can only deploy your units on the top half of the battlefield. The imperial guard armlet can only be obtained here and in the Northern Crater (Disc 3) still, there are at least 4 armlets more useful than this one, and they're found earlier in the game.
So you're going to spend money on a gigantic warfare simulation. The Fort folks merely want to protect the giant condor who is nesting on top of the reactor. The basic problem is this: Folks at Fort Condor are waging war against Shinra, who seem to be hell bent on conquering back their precious Mako reactor. It's easily accessible from Junon (though a river appears there later in the game). Fort Condor is located in the Junon area, near the back entrance to the Mythril Mine.