Dead Space 2

I found a nice way to kill Stalkers at your leisure in Chapter 6.

You enter a warehouse room, where you trigger about 6 or so Stalkers halfway across the room. As soon as you hear the Stalkers make noise announcing their arrival, run as fast as you can to the stairway you entered from. The doorway from the stairs to the warehouse room is a transition point in the game, and if you cross to the stairs, the Stalkers become disinterested and wander back into the warehouse.

Simply cross into the warehouse, wait for the Stalkers to run around and take a hidden position, and headshot them a couple of times when they peek out. As soon as you shoot once, back up through the doorway, and they will not run at you and attack (or will even turn around if they were already running your way). You can pick them off one by one at your leisure by repeating the in-shoot-out pattern.

Obviously, for this achievement, you'll need the Seeker Rifle, you'll need to aim (LT) and select the alt-mode (RB) before firing the killing shot.

This achievement is not super easy as Stalkers are really fast and on most difficulties will not die in one shot. I got this one the second time Issac faces a group of stalkers after killing the Tormentor. You are in a room with a bunch of orange shelves and there is both a stasis station, and throwable kinesis spears on the ground.
Once the Stalkers come out wait until one is peeking out from behind the shelves and use stasis on it, once it is in stasis zoom and try to hit the stalker in the head, I played on the survivalist difficulty my first playthrough and it took two shots to the head to kill him with no upgrades. If the stalker manages to get away just refill your stasis from the station on the wall and try again. There are plenty of stalkers in the room. If you manage to run out of Stalkers just exit to the main menu and try again. There is a save station on the wall as you enter the room before you go down the ramp.

Thanks to Calex dEUS who mentioned that if you're running low on Seeker Rifle ammo you can bump the difficulty down to casual and one shot them. If your worried about achievements for difficulty playthroughs you can exit back to the main menu after you get the achievement and re-load as there is a save point right before you go down the ramp into the room with the Stalkers. This will resume the game with your difficulty still on Survivalist or whatever it was before and ensure that you still get the achievement for your selected difficulty.

Chapter 9, after you get off the transport and enter a room full of boxes. Just wait for a stalker to run towards you, stasis it and shoot the head using the seeker in zoom mode.

Very easy, did it on my first try.

If you upgrade the ALT fire on the gun it can get all the way up to 100 damage per shot. This is strong enough to kill stalkers on Zealot difficulty with only one shot. Stasis could help if you are having any difficulty.

Also keep your back against a wall or better yet in a corner to make it easier.

EDIT: The black-colored stalkers starting on chapter 10 will take 2 shots on Zealot difficulty.

Buy a Seeker Rifle. Now look for a Stalker (the Necromorph who tries to avoid you and charge at you full speed when they think youre not prepared). Before it charges at you hold LT and then press RB (To aim then zoom in). Let it charge at you and before impact shoot it. It may not kill it right away but its okay, just get up and try again. The second shot should kill but Stalkers like to take turns if there is a group them so keep your eye the ones you damaged.

Remember: You can also freeze them with Stasis before impact then kill it (which is smarter).

Another easy place to get this is in Chapter 13.
There will be an area where 3 Stalkers jump down one after the other. Zoom in on them as they're jumping down, stasis them if it helps, and one or two shots kills them. Easy!