Story Related and Unmissable.
All you need to do is mentioned above.
As you progress through the floors you will come upon a huge hole. Jumping in you will be upon a big sandworm type boss. Kill it and you will unlock this achievement.
Tips:
*** Spoiler - click to reveal *** The boss has a few different attacks.
- The first is one where she stops and lays her eggs. The green blob-like things that pulsate. These hatch into a small larva that will shoot you with a sticky web. You *CAN* destroy the egg before it hatches, which is what I'd suggest doing.
- The next is when she spins in a ball and shoots a shockwave in all directions. These are easy to avoid as long as you keep jumping.
- The last is where she shoots across the room, she will then bury herself in the ground and a target will appear around you. After a moment, blocks will circle around you. You can't destroy these blocks and you want to jump over them as soon as possible, as she will come flying across the screen and hit you.
Apart from that, you will want to avoid her main body as this will also hurt you. The only part of her you can hit as well is her tail, which is easiest done when she is stopping to lay eggs or shooting across the room.
She will take a few goes until you kill her, and it does depend on your items and health when you get to her.
It's also wise to play it by ear. For instance, I said to kill the larva, however, if you have the item that allows you to ricochet your weapon from one enemy to another, it is best to let them hatch as throwing your axe would allow you to get an easy hit on the boss.