On top of the second tower(Data Processing Security Tower) you will have to fight a kind of small boss battle against a gladiator and two flying guys shooting rockets at you. Upon defeating them and firing the second laser, you will receive a message to return to the tram. Now it is IMPORTANT to save the game before entering the elevator. Pick the weapon you intend to get the achievement for(at this time you have collected all weapons), and enter the elevator. After the big strogg-beast breaks the glass fire a few shots at it and exit the level(its a really short sequence). Achievement pops up. Load your save file and repeat for all the other guns.