With the following method you can get nearly all of the weapon achievements in the game:
During Episode 3-2 when you are at the Coal Mine Museum, a short way into the level you will walk through a door and enter a building into a long hallway. Three Taken will drop from the ceiling and another will walk in from the left at the same time. Throw a flashbang at the group and you will get all four of them every time. This will also net you the achievement for killing four taken with a single flashbang, if you don't have it yet. Now reload the checkpoint. You will be right outside the door again. Walk back into the hallway, wait for them to drop and flashbang them again. Rinse and repeat to get all fifty.
You can use the same technique to get all of the flare gun kills here. Also, there are a couple of explosive canisters right where the Taken drop down, so you can get your Indirect kills here as well. This is also a great place to get the shotgun/hunting rifle achievement, depending on which one you are carrying.
http://www.youtube.com/watch?feature=player_profilepage&...