1 Don't place any units
Don't place any units on the mountain and start the battle.
2 Wait for enemies to climb
Wait for the first enemy to climb and reach the top of the mountain.
3 You will get an invasion alert
When an enemy reaches the top you will get an enemy invasion alert. The man inside fort condor will tell you that they are counting on you.
4 Fight the enemy
In such a case, you will have to fight the enemy with your party members as if it's a normal battle. See also how to let your Materia level up fast.
5 Defeat the monster and it's over
Once you defeat the monster, in a relatively easy battle, the invasion will be over and you won't have to do anything else.
6 You will only fight one unit
No matter how many enemies are attacking you, you will only have to fight one enemy.
7 Repeat this for every battle
Whenever you pass by fort condor you can do the same to stop an invasion. You won't pay any money if you apply that plan.