Ok heres the deal to get all the weapon achievements all you have to do is wait till after the second tower. After the data processing security checkpoint you will recieve an autosave called DATA PROCESSING SECURITY
this is the autosave your going to be using to get the achievements. Basically its the level ( if you call it that) where you get attacked in the elevator by the big strogg beasty and cant move. all you have to do is chose you weapon and wait for it to break the glass then shoot it
. The elevator will crash you go through a door and exit the "level". Acheivement unlocked, all in under 2 min. And can be done on any difficulty since you cant die.