For this achievement, you must shoot a helicopter pilot at point-blank range with the Magnum.
You must be playing as Jake in Chapter 4 of his campaign. The mission will start with a rather long chase scene which features you and Sherry on a motorcycle. When this long sequence finally ends, you'll be in an open area with a bunch of J'avo and the helicopter raining fire on you. You'll have help from Chris and the BSAA, though. When you finally clear this area, you'll come to an area similar to the one you were just in. The helicopter will return, but this time it will be targeting the BSAA on the roofs.
In this second area, the helicopter will occasionally drop off enemies on the roof of either of two buildings. Jake can get to the roof of these buildings, and you can climb up the rope that the helicopter uses to drop off the J'avo. But getting onto the helicopter's rope is not as easy as it sounds.
ANTICIPATING THE HELICOPTER
The hardest part of this, for me anyway, was judging which roof the helicopter would land on. I haven't found any patterns or specific tricks as to nailing this, sadly; for me it was literally just a guessing game. However, you'll know when the helicopter is about to drop off an enemy because it'll make a large distant lap. When it returns, it will proceed to one of the two ropes.
You could technically wait on either one of the roofs. This will most likely work, and if you're struggling too much with the achievement you can revert to this. However, standing in between the two roofs will also work. When you see which roof the helicopter is making a break to, you can make a dash for that room. Make sure you're sprinting so you get a running jump on the ladders!
WATCH OUT FOR THE J'AVO
The helicopter does drop off three J'avo every time it hovers above one of the two roofs. These J'avo can potentially knock you off the rope as you are climbing, which would really suck if you're having trouble getting on the rope in the first place.
If you were waiting on the roof when the helicopter arrived, you can try killing the J'avo before climbing the rope, as there are only three. However, you have to be careful; the helicopter doesn't stick around for long and you could miss your chance to grab the rope!
If you're playing on a lower difficulty, you can try climbing the rope without killing the J'avo; there's no guarantee they'll shoot you off the rope anyway, but on a lower difficulty, even if they do, they may not do a damage to get you off the rope!
RACING THE B.S.A.A.
Unfortunately, you don't have forever to do this. Chris and the BSAA are targeting the helicopter, and you must grab the rope before they shoot the helicopter down themselves! They won't shoot it down that quick, so you'll have a good amount of time. The helicopter will usually be able to make three or so drops before the BSAA shoot it down.
If they BSAA shoots the helicopter down, you'll have to restart the mission. Luckily, unlike Flying Ace and I Prefer Them Alive, this sequence isn't too late in the mission!
AFTER GRABBING THE ROPE
Grabbing the rope is the hardest part, but you don't want to grab it and miss your chance for the achievement, so know the QTE's you'll have to press. You'll be prompted to tap X to climb the rope (the faster you click, the faster you climb!)
Once you get to the top, Jake will flip himself onto the hood(?) of the helicopter. Switch to your magnum and aim down towards the driver. The game will prompt you to click RT to shoot, scatter the pilots brain over the helicopter's windshield, and net a pretty tricky achievement.