How to win fort condor battles easily in Final Fantasy 7?

Don't place any units on the mountain and start the battle.

Wait for the first enemy to climb and reach the top of the mountain.

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.

In such a case, you will have to fight the enemy with your party members as if it's a normal battle. (See How to defeat Lost Number – Shinra’s Mansion – FF7)

Once you defeat the monster, in a relatively easy battle, the invasion will be over and you won't have to do anything else.

No matter how many enemies are attacking you, you will only have to fight one enemy.

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. (See How to Catch a Chocobo in Final Fantasy VII (FF7))