Battlestations: Midway

There are pretty much three ways to complete this:

#1: Get a buddy, set up a boosting session, and sit in your sub for 15 minutes. Your friend quits, achievement goes pop, 30 gamer minutes burned.

#2: Do it in the single player campaign. There are just a couple problems with this method: 1) The only mission where you control a sub is on ???; 2) The mission begins with you controlling a destroyer. If this is your method you're going to need to rush through the beginning of the mission in less than 15 minutes due to the second caveat of this achievement "used this unit type [a sub] the most in the mission" just to keep the necessary time at a premium. Of course you can drag your behind through the mission, and then just sit in the depths for longer before completing your final objective. The end result: achievement popped, 15+ (likely more than 30) gamer minutes burned.

#3: This is probably the best method (I am kind of impartial though since it's how I did it). Do it in the first Sub Challenge mission! It's kind of difficult to do (but not really since I'm going to provide you a step by step guide), but you'll get the achievement in the minimum amount of time (about 16+ minutes depending on how much certainty you have on your method of keeping time).

When the game begins, you are tasked with identifying two particular ships (the ships to be identified are random). To "identify" the ship, you must target the ship, just passing, or hovering, your binoculars over the ship is not enough. The easiest way I found to take care of this is to quickly hit (a) while slowly panning your binoculars over the convoy. As you scan the convoy you'll see the reason for your haste, as there's an enemy destroyer bearing down on you.

Once you identify the two ships, you'll get a new mission, to sink them. At this point, you want to dive to crush depth, and head due west. While at crush depth, you won't be able to target surface ships or even see your target ships, but you will still be able to look up and see the bottoms of the hulls on the surface of the water. Being at crush depths, the destroyers will be unable to locate you or damage you with depth charges. Even though you'll be slowly taking damage while at this depth, it is pretty insignificant and you'll be able to last the entire 15 minute plus to get the achievement and still be above (or close to) half life.

Once you get into the midst of the convoy, rise back to periscope depth, identify the mission targets, and sink the nearest one. It should take two torpedoes, but just launch four to make sure. Once the torpedoes are away, identify your next target, BUT DO NOT SINK IT! Once the second target is sunk the mission will end.

At this point, you want to return to crush depth and shadow your remaining target until the time expires. During the course of the mission yo are going to need to resurface for air at about 10 minutes mission time. The destroyers will be so far behind you at this point that you should be able to fully refill your air supply before they can close to artillery range, and the supply ships are unarmed.

Never the less, once you have enough air, return to crush depth and wait out the remaining time before sinking your final target.

I don't know how accurate the timing is, or when the timer starts. I checked my clock when I was able to maneuver the sub, and waited until 18 minutes had passed before releasing torpedoes on the final target