To obtain this achievement you'll need to kill 4 hunters at the radio transmitter. Since you'll need to learn how to take out large numbers of hunters without ammo for the final mission, this is an excellent opportunity to get some experience in that area. Alyx will suggest driving up to a radio transmitter location on a hill. There are a number of buildings here, but the action will take place in the central building. This building has 2 rooms: a main room you can walk into, and a lab that can only be reached by walking up a dirt ramp and entering through a crack in the wall.
First, prep for the fight. There are numerous tires distributed throughout the area. Collect 3 of them and drop them into the cracked wall (don't enter the lab yet, as you won't be able to get out). Find 3 more and drop them in the main room next to the locked door to the lab. Now go through the crack into the lab. You'll need to drop into the basement and solve the physics puzzle in order to turn on power to the transmitter and the lift. Once you return on the lift, a pair of hunters will be triggered.
Pick up a tire with the gravity gun and wait for the hunters to demolish the wall between the 2 rooms. Wait for them to approach the lab and start pummeling them with the tires. They will back up. If you lose tires across the room, use the tires you placed in the 2nd room. The first pair will take 2 to 4 hits each to bring down. You'll probably pick up the Payback achievement here.
After the first 2 are down, collect your tires and drop them in the enclosed area outside the main room. Don't go outside until you're ready as that triggers the second pair. When you're ready, step out and trigger the hunters. Walk back into the enclosed area and wait for the 1st hunter to show up in the doorway. It should go down in 2 hits, and be followed by the second hunter. Repeat.