You'll find Dust Man hiding in a factory. This is one of the more difficult stages in the game, so be ready to die a little bit if you're not constantly on your guard.
From the start, head right along the ledges. You'll soon run into an armored, red enemy that zips along toward you. He follows set paths, so leap over him, then leap over him once again as he turns around. When his back is to you, let loose with arm cannon shots to defeat him. As you continue right, you'll also run into the hardhat enemies, which are only vulnerable when they peek up long enough to either move or fire a spread shot. They've been around since the original Mega Man game, though, so you should be used to them by now.
As you continue, there are small gaps where bottomless pits await you, and you'll want to leap across those. However, enemies tend to fly up from those openings, so you should first lure out an enemy at each pit, shoot it, and then attempt the jump, rather than getting knocked into the pit as a result of unfortunate timing.
Pressing right, you'll find more hardhat enemies, and more of the shielded red guys, until finally you come to a wide pit with green-colored walls behind it. You're safe to drop down this pit, so go ahead and do so.
In the next screen, you'll be on a high ledge. You can drop off its left side to reach the lower ground level (or slide left to collect the large energy capsule if you need it). When on the lower level, start charging your mega buster and head right. You'll soon encounter a new robot enemy, which looks a bit like a glorified, chubby pogo stick. It has spikes at its base, and is quite large. Pelt it with shots until it explodes, then continue toward the right.
Now you're at one of the stage's trickier parts. Small blocks will fly up from the bottom of the screen to form large blocks in the air that you can use to proceed to the right. Hop along these as they materialize, hitting any flying robots with careful shots from the arm cannon. Don't let them hit you, as they do a lot of damage and may knock you onto a row of spikes. Also, don't let the slowdown that occurs in this area cause you to misjudge a jump. If you do things right, you'll cross over to a ladder leading up to the next area of the stage.
At this point, you're in a room with ledges leading up to a ladder to the left. Fliptop will descend them and toss you a power-up, hopefully a large energy capsule so you can refill some health. When he's gone, climb the left ladder into the next area.
Now you'll need to work toward the right while the ceiling above raises and lowers itself. There will be a hardhat enemy or two along the way, but they're no real threat. There also are wide bunches of destructible blocks. Take care of those with arm cannon shots and clear the way to proceed. Soon, you'll come to a place where the ceiling gets low enough that only a slide is safe. You now must venture right as the ceiling starts to rise, then rush back left as it falls. Each movement toward the right should allow you to clear away part of the block obstruction that lies ahead. It may take three of four such slides to clear the way, but don't allow yourself to rush or you'll likely end up crushed.
Once the way is clear, just keep moving right and you'll emerge safely from the compromised ceiling. From there, climb up some steps to where another enemy waits. This is the red guy that fires shots straight ahead and lobs them. You first encountered him in the Bright Man stage. When you've beat him, move forward to climb a ladder up into the next room.
Here, you'll see a pit of spikes, with a ladder leading up from the left side of the screen. There are two platforms you can use to cross over the spikes. Hop onto the one closest to you, edge forward, and let loose arm cannon fire to defeat the hardhat enemy that waits on the second ledge. When he's gone, hop over to that ledge, and from there to the ladder.
Climb up into the next room and head right, where another of the pogo-like enemies awaits. Once you've defeated him, you can continue walking right to find the gate that leads to Dust Man.
NEXT: Dive Man