Dr Wily. When will this guy learn to quit?
Either way, it's up to you to bring him to justice yet again.
At the beginning of the stage you'll find an Energy Tank. Get it, kill yourself until you get game over, and continue. When you continue, get the Tank again, and repeat until you have nine Tanks. Time consuming, but it will make the fights to come much easier. Be sure to save several of them for the final bosses.
From there on out, it's a pretty straightforward level, with the main challenges coming from two unusual sections.
First is an area with conveyor belt wheels over deadly pits and enemies. You have to be careful because of a ceiling lined in spikes. As usual, Rush Jet an alleviate some of the difficulty.
The second challenger comes from an area that looks like it was ripped straight from Dust Man's level in the previous game. The smasher ceilings have a ton of debris below them that needs to be cleared away with your arm cannon. Clear it and move forward, shooting, running, and sliding to get through this tough area.
The first fortress boss is a weird enemy who has a body in three segments. When you shoot one of the bottom two segments, it will fly out at you. The spiked sides of his body are harmful, but you can stand safely on the upper parts, using them as platforms. The top is the "face", where you have to shoot him to do damage. Technically, he's weak to the Crystal Eye, but since you need to fire shots just to get the platforms to come out, I recommend sticking with your regular weapon. Fire a shot to bring out the first section, climb up, repeat on the second, and then unload on his face. It takes some time, but this guy really isn't too touch to deal with.