Half-Life 2: Episode One

For this achievement you have to kill five enemies with the same energy ball. The easiest place to get this is on the chapter "Direct Intervention" after you get past the two stalkers. You'll have a stream of energy balls to the left of a doorway into a hall. Grab one of those and wait in the hall until 5 of the combine soldiers come running in and shoot it at them. If you end up not getting the 5 kills you can reload and try again. I also cover this achievement as well as all of the others in the game in my video guide linked below.

For this achievement you'll need to make it through the entire game while only shooting one bullet. Grenade, crowbar, rocket, and Grav gun kills are all okay still. There is a part during the third chapter “Lowlife” where you’ll have to shoot a lock and that is the ONLY time you’ll be able to shoot if you’re going for this achievement. I also covered this achievement as well as all of the others in the game in my video guide linked below.

This guide will explain how to complete both "Think Fast!" and "Conservationist" in one go.

The quickest way to do this is to start a new game on Chapter 2: Direct Intervention. Set difficulty to Easy.
Walk through the door right in front of you and walk slowly up the hallway. A group of five Combine will appear.
The Combine Elite on the right will shoot an energy ball at you as you approach slowly.
Grab this energy ball and shoot it back at him. (At this point "Think Fast!" will unlock)
As soon as you release the ball and hit an enemy, grab it again and repeat for each combine in the room.
This will unlock the "Conservationist" achievement.

For this achievement you'll need to light 15 zombies on fire while playing through the game. Once you get to the chapter "Lowlife" I'd recommend using any flares you find to try to get a kill or two in the beginning areas to make this a bit easier once we get to the elevator area. The big area where you’ll get this done is once you find the powered down elevator and have to follow a wire to a fuse box. After you activate that it will start spawning a horde of zombies that will spawn and attach. There are plenty of flares spread throughout this area that you can grab and light these on fire until you get the achievement.

I also covered this achievement as well as all of the others in the game in my video guide linked below.