Phoenix is an easy HVT, though he might get you on your first try. This guide will show you how to do this with ease, but be warned, you need to be a good shot for some of these parts or at least play tactfully.
Start by selecting your character. Since I am leveling all of mine still, I just use the recommended. Here is the build I used, but you only need the pistol (triangle) to finish this mission. I strongly suggest this shotgun, since I was sniping people with it no problem.
After launching the mission, head to the right. There is a person to the left, so it’s really the obvious choice. You will encounter a C4 there, but you can disarm it by walking up to it. Once it’s disarmed, sneak forward. I had no issues making it to the end of this area unseen.
This is the end of the first part. Turn around and you’ll see C4 on the door.
You’re welcome to disarm it, or you can blow it up. The latter will alert the place, but you can run out of there and it won’t be an issue. Keep heading forward till you find the next room.
This room is covered in mines, so expect a lot of death. I strongly suggest just alerting the place. If you go that route, three guys will appear on the lower level and two will be on a ledge. Take out the two above, then just set of a mine killing the rest. Head for the exit, but be aware of mines.
Here you can see various mines at the end. Make sure these are taken care of or you will most likely die.
In the next area, you should find a room like this. Hug the wall and just shoot this guy in the face. If you alert him, someone above him will start shooting at you. Keep this in mind. You can just wait there will the area is safe.
Repeat the same method as before. As you can see, I took out everyone that was coming at me here. Once it’s safe, you’ll find the HVT here and he should easily die. Once hes dead, head to the extraction point for your trophy.