-For those of you having trouble with this achievement, or who would rather read a guide than load videos, I'll do my best to paint a detailed mental picture/ guide for you. Keep in mind, I wrote this much on purpose, not by accident. If you only care about the actual firing of the minigun/ killing 20 enemies, feel free to skip forward to 'Part Three' now.
[Part One: Basics]
-This achievement is easiest on recruit difficulty, mostly because the enemy AI is far less aggressive, meaning that your team wont have to kill as many in self defense, leaving more alive to be mopped up during the achievement. That being said, this solution will work on any difficulty, just take it slower and be more tactical in Hardened / Veteran modes.
[Part Two: Getting There]
-The most simple and convenient way (my opinion) to unlock this achievement occurs near the beginning of the level, after crashing through the large steel gates, and exiting the Jeep. -Immediately after gaining control, head left and follow the edge of the map, sticking close to the wall. During this time Soap will say: "Slot these b**t**ds fast!" - Disregard that, and rather, DO NOT KILL anyone during this time. If you do, there are less enemies to kill when the chopper comes in. -Keep moving along the wall, then down a wooden ramp. -Look for the light blue shipping container, it's the closest one to the bottom of the ramp. -Run forward, past the couple enemies if you can avoid them, and stop somewhere by or between the container and the wooden pallets. After a couple seconds, you'll hear Price say: "Waarabe's compound is at the end of the road! Move!" - Again, disregard this order, and stay exactly where you are, behind the blue container. If for some reason an enemy is attacking you, and cant be ignored without dying, kill them. There shouldn't be a reason to kill more than one enemy during this time period, and ideally, none. -Price yells: "They're targeting us with mortars!" This is true, but yet again, disregard it, as the mortars can't hit you back here, even while you're standing still. Right after, Soap calls out: "Nikolai, we need air support!" and Nikolai responds: "Moving into position now." -Pause the game.
[Part Three: The Achievement]
-While it isn't necessary, I highly recommend turning your controller sensitivity down 1 or 2 from the setting you use for your main gun. It can be very helpful in controlling the chopper gunner, as it aims very quickly-- which can cause you to over-aim since the helicopter is moving as well. I found best results with the sensitivity around 3. - Resume game, and press --> (right) on D-pad to enter gunner controls. - As soon as you gain control, hold the trigger down so that you can hit targets as soon as possible. This particular minigun doesn't overheat, so you might as well be firing every second. Don't worry about firing over your friendlies *too much*, so long as you don't -focus- your fire on them/ kill them, you wont usually get penalized for it. - There will be a 'front line' of enemies spread left to right, so rather than fire straight ahead as you move forward, sweep your fire across them. This way, you will also be more likely to hit explosive barrels, which will each kill multiple targets at once, saving you time. It's difficult to spot individual barrels, so just know that they are usually close to groups of soldiers. There are a lot of them on the ships located to the left of the enemy 'front line', where you should fire next, and there are clusters of them on the building rooftops, where you focus on third. Aim near the water towers/ other structures on tops of buildings. Sweep across them, up and down as you approach, and left to right while you are above them. Then focus fire more -into- the buildings, where the windows are. Finally, pick off as many stray enemies in the streets as you can before the gun fades out. If you got 20, the achievement will pop before you put the control screen all the way down. Otherwise, act quickly to restart checkpoint and try again, because you only have a couple seconds before Price will speak and you'll have already reached a new checkpoint, forcing you to try and get 20 kills on later runs, which is much harder (in my opinion) or start the level all the way over. I hope this guide has every detail someone would need for attempting unlock on the first gun run. If there is anything I should add, entire paragraphs I should remove, or any mistakes, let me know. I will edit this tomorrow as I have more time.
If everything I just said only confuses you more... Try the condensed version: spray and pray on the first enemy line, then the boats, then building tops, then building centers, and finally shoot individual targets on the roads. clusters of bad guys first, and save the enemies out in the open for last.