This achievement aligns with the requirement to unlock the Refreezerator pod. It wasn't until the third time I completed the objective of not having the crystal touched that I realized what I was doing wrong. The game keeps up with your crystal via lost dust. This includes dust you may spend with a merchant. So, if you meet a merchant that buys and sells for dust while you're going for this achievement, DON'T BUY ANYTHING! This is what was messing me up.
It doesn't matter which pod you use, but the Escape Pod or Library Pod is easiest.
As far as protecting your crystal, the greatest dangers are the doors right around the crystal itself. I would rush a door with all four players, then set up guns/neurostun, etc. and leave someone to guard that area. Always bring your four people back to the crystal before you open a new door leading off the crystal. It can help, too, having someone like Kreyang along who has an ability that slows enemies if you get in a bind. Good luck!