Under Wily's Toilet!
After our exciting gauntlet of trials in the last stage, we're now about to have a very short and easy stage. I guess the devs wanted to give us a break or something? Anyway, after defeating Copy Robot, we're again transported to the next area of the stage. I don't exactly know where we are or how we got here, but we're here, and we're going to keep searching for Dr Wily.
Unlike the last stage which took place mostly outside, this stage is a more dreary-looking interior with olive-colored background and weird speckled salmon walls. There's a series of rooms that are nothing but a bunch of Adhering Suzys, with a couple Screw Drivers thrown in to mix it up a bit, I guess. None of this is particularly challenging or difficult, especially since we have access to the Thunder Beam which allows us to take out all of the threats from safe spots. The biggest test here is a test of patience, as it can be a little boring waiting for Suzys to move to a place where you can hit them, and you might just try to jump past them and get bumped. Even then, your chance of dying during this segment is pretty low.
After we fall down through several passages of Suzys, we enter a very long corridor with seemingly no danger. That is, until suddenly water rushes in and fills up the place! I'm actually surprised I was able to get a screencap up of it, because it happens very quickly. Once the water has filled the screen, Rock is constantly pushed to the right. The force is so strong that even if you try to run to the left, he'll still move slowly to the right. And actually running to the right will make you go very fast.
A bit after the water fills up the tunnel, some Pepes will start flying out from the right side of the screen. They're very easy to just hop over, or you can just repeatedly shoot and you will undoubtedly hit them all, avoiding all danger. After a while, the Pepes will stop, and after another brief moment, we'll reach a Killer Bomb segment. If you're still firing because of the Pepes, there's a good chance you'll shoot a Killer Bomb and then the force of the water will push you into the explosion, causing you to damage yourself. I actually forgot to stop firing after the Pepes but somehow managed to avoid damage by getting lucky and shooting the Killer Bomb just as it appeared on screen, giving the explosion enough time to dissipate before Rock reached it.
Of course, you can just keep hopping over the Killer Bombs to avoid them fairly effortlessly. Eventually, we reach the end of the tunnel, which opens into a slightly larger little room that turns into a smaller tunnel. This is a bit of a "gotcha" moment for the player -- if you just keep running forward like you were, you'll smack right into a Killer Bomb in the little tunnel. If you try to jump, you'll just press against the right wall and the Killer Bomb will hit you as it rises. The key here is to start running left to slow down Rock's forward momentum for a bit and "wait out" the bomb. Once it floats upward, run under it and into the little tunnel.
And with that, we're already at the boss of this area! Yes, this stage is incredibly short and easy. At least up until this point. This boss can be quite tricky if you're not prepared, though. I don't know if we were supposed to have fallen into Wily's sewer system or what (ewwww what if his poops are in here?!) but of all places to hide some crazy patrol robot system thing...
Meet CWU-01P. This thing is not just a guy in a bubble, but an entire system of guys in bubbles. The battle is pretty simple -- one of the bubbles will come out of one of the three passages at the top or either side of the screen, and then hover around the edges of the arena, firing slow little bullets at Rock. There are two big dangers here: one is that these things are HUGE, and Rock can't jump over them without the help of the blocks in the center of the arena, so just hopping over one takes a bit of effort and timing. Secondly, each time one bubble is destroyed, the next bubble that comes out moves faster than its predecessor. The final bubbles are ridiculously fast, to the point where it's nearly impossible to make time to jump over them.
Rock needs to shoot them several times (I think 10?) with his Rock Buster to destroy one, and the only weakness they have is to the Super Arm, meaning to utilize their weakness, you have to pick up the blocks in the center of the stage, removing your "stairs" for jumping over the bubbles.
But if you know what you're doing and you're willing to use the Super Arm, it's not so bad at all. The first few bubbles will be very slow, so you can easily take them out with the Buster, maybe getting hit a few times. Then save the blocks until the last four bubbles, since you can destroy one in a single hit with a block, so you don't really need to use them to jump over the bubbles anyway.
The problem is they will appear and start moving very quickly, so you need to already have the block in hand before they even appear, and be prepared to jump near the middle of the screen and throw it in the right direction so that you don't miss as soon as one of the bubbles starts moving.
In the end, it's not too hard, and you can even afford to get hit a few times, making this the easiest stage in the game. And yes, they were really giving us a break after the last two stages because the next stage is going to be a lot tougher...