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First build a support station, and then surround your base with electric fences. Once you have done this, you can start drilling on a large scale. It doesn't matter where you drill, but be aware that there are rock monsters that will eat your crystals. So it is best to get a small digger and dig the right hand side (east) wall. You should give half of your rock raiders weapons (or all if you collect enough crystals to have a couple of transport trucks).

The crucial thing with this level is to never allow yourself to have 10 energy crystals stored. This is necessary because once you get 10, the slimy slugs come and then the difficulty level increases by a large amount. The basic idea is to switch off "collect energy crystals" on your priority list, manually tell your rock raiders to pick up crystals, and drop them inside the protected area of your base. Then, once you have collected 45, switch energy crystals back on as first priority, put your rock raiders on red alert and then defend against the slugs until all 45 have been deposited.