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The collectibles on this map are in the form of batteries. Shoot 10 to unlock this achievement. Refer to "Catacombs Cash" in the main guide for more information.

The collectibles on this map aren't available all at once. You start in Sector 1 and then when you complete the wave and get to the exit, you spawn in one of 9 other areas. There are 10 different non-boss areas and are completely randomized, so if you are following the video below, it might not be in the same order as the game you are playing. This is similar to the map 'The Descent' from the base game.

Each area has 2 collectibles so the earliest you can unlock this achievement would be Wave 5. It is recommended that you play a Long game (10 waves) on Normal as the more powerful enemies don't show up until later waves.