Kao the Kangaroo

The Crab King is the final boss of the DLC. You'll need to complete all four levels in Easy mode, grab the rune at the end of each, open up The Eternal Sea level with them, and progress through this entire level to reach the Crab King at the end.

This boss fight is a bit of a pain, especially because you don't have much health (relative to how much you probably had in the base game). You can beat the boss readily in four cycles, and here's how each cycle will work:
 

• You'll be targeted on the ground, and it'll turn red. You need to roll out of the way as soon as the target starts to shrink and turn red so you don't get hit by a laser. Do this at least ten times until it finally ends. Then, you'll see giant crosshairs on the ground for where crystals will fall and smash. Simply avoid them as you approach its claw. Run up to either crystal on its claws and attack it nonstop until it raises (you should be able to take off 25% or so of the total boss health).
• The second cycle starts with three of the same laser attacks as above, but they usually come pretty quick, so be prepared to roll almost immediately after the previous roll. After three or so laser attacks, it'll shoot lasers at the ground somewhere nearby, and this will send a shockwave across the arena. These are very simple to just double jump over to avoid. After around six or so of these, his crystals will drop down again. Attacking the same crystal should destroy it and hit the halfway point.
• The third cycle (and second half) starts with a bunch of little golden crabs dropping down. They're immobile, but don't touch them or they'll hurt you. Cycle one now repeats, with the laser attacks you need to roll away from. Just make sure you don't roll into a golden crab. Just go back and forth in the same small area to avoid them. Then, avoid the falling crystals and go attack his other claw.
• Lastly, the fourth cycle is just the laser shockwave attack over and over, but with two flying masked bird enemies. This cycle doesn't end until you kill both bird enemies. I found it best to circle around them (this avoids their attacks and keep them close), and jump to avoid the shockwave, using that time to get in an aerial attack or two. Once you beat both, avoid the falling crystals and finish off the second claw to beat the boss.