This is easy to get just from normal play, especially if you're being thorough. Be sure to take out all the enemy squads (some will have weapons, some will have pass codes). Check your map for the location of terminals. A little ways in, you'll probably need to revisit prior areas to find new things. Just make sure you've collected everything from each area by defeating the squads and accessing the terminals.
You should end up with the Javelin, Geyser, Phalanx, Halbred, and Sniper - five sub-weapons to unlock the achievement. This can all be done before the point where you need to find an unused Raptor.