First up go to Spartan Ops and chose the first Episode (Departure) and the fifth Chapter (Core). You can put the difficult to easy just to get it faster but you can also gain the achievement on any other difficulty.
Now play to the point where you need to push the first button in this mission. Now standing right in front of it, like you want to push it, go near the lower wall to your right and look down to the lava. There you should see a little box on a rock. You can't jump down so just shoot it. Because it's hard to see a photo will do better at this place :)
A really big thanks to Carl Corny for showing me this thing!
(Anybody in the game should get the achievement if just one shoot it. If you find any mistake pleas comment. I don't own the video.)