Finally, you've reached the last stage of Protoman's Castle. It's quite short, but the battle at the end will eat through your resources.
You find yourself at the base of a vertical shaft, with spikes positioned along overhead ledges. There are walls comprised of crumbling clay, and you can shoot those to destroy them. When you do, the architecture drops slightly, equal to a single block.
To ascend the tower, then, you'll need to carefully shoot blocks. There's no rush, so make each shot one block at a time and watch out for the spikes. You also need to be ready to shoot and then slide to the side to avoid getting crushed. Start by shooting the three blocks to your right, then take care of the lowest block to your upper left. Slide through the opening, then face back toward the right and destroy the two columns currently positioned on the lowest tier.
Make a tiny hop so you don't hit the spikes overhead, and next take out the three-block column just to your right (while standing to the right of the overhead column of clay blocks so that they don't fall on you).
Now you can take care of that high column of blocks, one at a time. When the last of them fall away, you'll have a moment to quickly move left before you are crushed. Then you can take out the remaining block to your right before you tend to the column of blocks now positioned to your left. Once those are out of the way, then you can finally head through the gate to the area boss.
A scene unfolds, during which you meet a robot who appears to be Protoman. However, that's all a ruse, and a short-lived one at that. Your true friend appears and unmasks the villain, and then you're left to do some fighting.
Your new adversary will use a combination of the attacks you faced while defeating the last three bosses in this castle. He's agile and he hits hard, plus sometimes he has shields that prevent him from taking damage. If you want things to go your way, you're going to need to focus primarily on taking as little damage as possible.
To help with that effort, summon Beat. Your flying friend can swoop around the area, dealing damage to your foe while you focus your energy on avoiding attacks. You're still likely to take a few hits, and each one of those hits takes off somewhere around a quarter of your life meter, so don't be afraid to use your E-Tanks if needed.
Of course, that last bit of advice comes with a word of caution: once you commit to using your E-Tanks, make sure that you actually win the battle. There's no way to easily refill them (that will come another stage from now), so you have one shot to do the battle right once you really go for it.
If this is your first time through the stage, you should give yourself a few practice runs if you think you need them before committing your resources. Otherwise, you could find yourself stuck on the wrong side of a battle you have little chance of winning.
NEXT: Dr. Wily's Lab