You need to put the Laredo (submarine) in a support position. Just kill 40 enemies while they're stunned.
I advise doing this on the last mission because there is a checkpoint near the end of the level, and you can easily kill 5+ enemies on this spot. Then, kill yourself with a grenade to reload checkpoint, rinse and repeat.
"Out of 10,000 feet of fall, always remember that the last half inch hurts the most"