OVERWHELM

Much like the One Down achievement, you will need to deposit 3 crystals into the crystal room at the center of the map. They do not have to be all at once and I highly recommend turning them in one at a time.

After defeating any bosses, they will always drop a crystal. Bring the crystal back to the crystal room (the center room where you dropped down from your horse at the beginning) and it'll unlock the achievement. ).

If you manage to die and you have a crystal on you, the crystal will then drop on the ground where you died, and you will have to go back to that spot to retrieve it. I would suggest turning the crystals in after each fight so you don't have to worry about trying to recover them if something were to happen.

Here is a pretty efficient way of defeating the bosses and bringing a crystal back to the crystal room:
Order of Bosses to fight
1. Kraken (Bottom right)
2. Lizards (Top right)
3. Worm (Bottom middle)
4. Skullbat (Top left)
5. Bush Spider (Bottom left) - Easiest to fight for it's ultimate form.

Once you defeat any of the bosses you can grab their crystal. You'll need to deliver these crystals to the center of the map in that machine that you first saw when you dropped down the hole. Deliveries can be one at a time or you can stockpile them and do it all at once.

Only thing to keep in mind is that each time you die while holding crystals, they will drop from your inventory and remain where you died. This is especially annoying in the underwater section as they will sink to the ocean floor.

I would recommend fighting the "harder" bosses first for these: Kraken, Twin Lizards, Tunnel Worm. You can however go in any order you want as all bosses are available from the start of the game.