Fracture

This is going to be a pretty hard battle unless you know the trick.

The youtube video posted in the other solution is helpful but after trying to replicate it for half an hour with very little success the following trick worked for me on my first try.

After the start of the level lower the terrain in two tunnels/pipes, run through them and enter the large room containing Sheridan. Follow the same path as described in the video above, follow the left side of the room, lowering the terrain. Here is the important part...jump onto the ammo box with your feet facing forward. Apparently this is a glitch in the game where Sheridan absolutely cannot reach you.

He runs right up to you but will not be able to hit you. Shoot his groin and face until they turn red. Each time you finish one of these areas he will summon pillars to recharge his health. You need to destroy these pillars. Some people recommend using the Black Widow you found at the beginning of the level to destroy the pillars. However it took me so long to aim properly and detonate that Sheridan would always regenerate. I recommend using your SMG instead. This weapon will destroy even the furthest pillars within only a few shots.

After you've done this twice, it's time to destroy Sheridan's back. Aim slightly below his left eye/head. He will summon creepers after every three stomps (it varies slightly) where he will expose his back to you. Keep firing at him and you can beat this level without ever even moving from the ammo crate.

this is pretty easy until you have to fight the end boss but all you have to do is follw this video and you should be golden then the achievement is yours. I didn't make the vid so dont email me asking for tips just follow the vid.

To add to the video above, I have a solution where you don't ever have to leave the ammo box and still kill Sheridan.

Following the guide above, get situated on the ammo box and fire away until he can't heal himself anymore.

Sheridan will start stomping on the ground three times, then punching the ground in a successive pattern, with a change in this pattern happening every once and a while.

When he finishes his three stomps and goes to punch the ground, jump straight up and shoot the green spot on his back, continuing until he is dead.

This may be tedious, but it saves you the trouble of having to run and dodge to shoot the spot on his back. I hope this helps those who use this.