The French used this tactic at the enemy.
This tank as a special machine gun that fires 1000 round/s at the enemy.
Should we use the plural "enemies" or is "enemy" fine? Why is that?
In the context of a particular war, singular "enemy" is correct.
The plural form "enemies" might be used if talking about the use of the tactic or the weapon against various other nations in multiple wars.
The preposition should be "The French used this tactic against the enemy."
The preposition "at" is correct in "...gun that fires 1000 rounds/s at the enemy."