Water! Jump in and head left. Jump over the holes and take out the Fisherns. Then head up the walls (taking out the Barite Lastars as you go) and out of the water. Take out the Garakuta robots and more Barite Lastars and head back into the drink.
Use the platforms to cross the spiked pits. You'll have to climb over the top of the third one and onto the right side, then either ride it across the pit or jump and air dash from it.
If you don't see a floating platform in this section with spikes on the walls, wait for it to show up. Obviously, don't try to climb the spikes. You wouldn't have made it this far if you were in the habit of doing that. There are flaming pipes on the right and left walls. Grab the sides of the platform to avoid them. Hold the dpad towards the platform while you jump and you won't jump far enough away from it to hit the spiked walls. (Don't dash jump.)
You'll need to get across the spikes. The lame way is to let one of the bats touch you and use the resulting invincibility to quickly hop into and out of the spiked pit to get across. The right way is to just use the Crystal Hunter on one of the bats and use it as a stepping stone.
If you clear the blocks here with a Spin Wheel, you can air dash and use a charged Speed Burner to get across the pit to a 1up. It's... not really worth the risk, though. Instead, head down, destroy the Cannon Driver, and head into the boss door.
NEXT: X-Hunter Stage 3