My friend, who doesn't know English, is trying to find a movie.
An alien species is in a war, and has allied itself with humanity. The aliens supplied the humans with spaceships, but they used human technology on land combats.
The technology from the humans was so primitive compared with the bad guys' that it ends up giving the aliens an advantage in the war.
The enemy aliens' shields were made to repel lasers and photon cannons, not bullets. They were also unable to track the humans' radio waves and the heat from diesel engines.
The good aliens win the war due to this.
It seemed to be an American production and it's not Aliens.
Update: My friend has just remembered it was most likely from the 80's.