Another achievement that is very easy to unlock.
Level 1 has 7 crystals. If you completed the level you should have:
How I proceeded to unlock the unlock this one is by completing level two in its entirety, which isn't too hard to complete. Push the boxes in front of you and collect the one crystal in the top right, then use the two boxes in front of you to create a bridge across. Once the level is complete, you'll have 12 crystals under your belt and will need to complete level 3 to get the last collectable achievement:
(Note to anyone reading this: can you confirm that you can replay level 1 and it will still count the crystals, even though we picked them up already? If this is the case, you'll only need to play the level three times to get crystal hunter IV. Genuinely curious and I'm sure it would save some time. I ended up getting the achievement half-way through level 3.)