Can be done on the easiest difficulty. In level 1-1 get to the area with the zig zag walkway. Walk across to where the next door is and on the right wall before the door there is a secret panel. Press A near the discolored wall to find the secret door and go in. There’s a small hallway that leads into a wide open area with a 200% armor in the middle of a poison pool. You won’t get the achievement until you complete the level. See the "HOARDER" achievement for a walkthrough on getting 4 easy achievements in one level.