You're going to want to wait until you're ready to power through the rest of the game before starting this stage, if you're not playing a version with save states like the Wii U Virtual Console version. You can't save your progress from here with a password. Collect all of the power ups besides the enhancement chips before entering this level.
Head right past the various recurring enemies. Notice that these Head Gunners are still the Customer type. Apparently Dr. Doppler has a stash of these.
You'll come to an area with a spiked ceiling. Sections of this ceiling will jut downwards. Don't jump into the spikes. Air dash to make it across the pits. Once a section drops down, it will stay down forever, so don't worry about it moving again. There are more Head Gunners here. Keep going right and wall jump up to the ladder.
These sections of the walls will close in on you. Quickly wall jump past them. Go up the ladder and to the left. Take out the Hamma Hamma and go up some more. It's miniboss time.
A spiked ceiling will gradually drop during this battle. If Zero is still alive, he'll show up and help you by stopping the spiked ceiling before it crushes you. If Zero is out of action, you'll have to defeat the boss before it crushes you. REX himself attacks by firing homing missiles and regular shots. He actually doesn't take that much damage before he goes down. Just shoot him with charged X Buster shots. If Zero isn't there, you'll have to stop the collapsing ceiling yourself. Head over to the right and use the Triad Thunder to destroy the device there.
Go right and jump down past the Caterkillers. You'll reach some Walk Blasters. Remember how you can use the Frost Shield to make these guys drop health? Well, you're going to want 100% of your health meter full in about five seconds. You can just use a sub tank if you have one. Trust me, you won't be wasting it. Even if you're only missing a tiny sliver of health, max it out.
Gold Enhancement Chip
Head down from the Walk Blasters and go right until you see a hole. Grab on to the left side of the hole and slide down. You'll go into a secret passage and find an upgrade capsule. This will only be here if 1) your health is full, 2) you didn't take one of the other enhancement chips, and 3) you have every other upgrade and collectable besides the enhancement chips. This chip will turn your armour gold, and give you the benefits of all four other chips. This means your health and sub tanks will replenish while you're standing still (which is why it was ok to use a sub tank a second ago), you'll be able to air dash twice, your damage taken is greatly reduced, the shield that appears after you take damage lasts longer, and you have a new weapon that will allow you to continuously fire charged shots. You are good to go! From now on, don't bother to save your sub tanks unless you're warned to. You'll always have time before or after a boss to refill them with your new chip. The Gold Chip won't be saved with a password so you'll have to collect it every time you play.
Go right, being careful not to get hit by the spiked balls. Take out the Drill Wayings to clear a path. Go down and destroy the Caterkiller and Hamma Hamma. Climb the wall and enter the Boss door. Which boss you fight actually depends on whether you destroyed Bit and Byte or not.
Godkarmachine O Inary
Uuuuuugh. What a stupid, stupid name. This is the boss you'll have to face if one or both of the Nightmare Police (Uuuuuugh, what a stupid, stupid name) survived. Look how ugly it is. Only its top half can be damaged. It's weak to Ray Splasher. It can shoot out its hand, which will grab you and slam you into the ceiling, then hold you in place so it can punch you with its other fist. If the grabbing hand misses, it'll still send out its fist, but you'll be able to avoid it. Its beam sword shoots a shockwave that will split into two smaller ones which travel up and down the wall when it hits it. If you're high on the wall, it will hover up a bit to aim its sword at you. It'll go down pretty quickly with the Ray Splasher. You'll have the opportunity to refill your sub tanks before the next level if it comes to that.
You'll have to fight the Press Disposer if you finished off Bit and Byte when you had the chance. This is the easier of the two possible bosses. Ray Splasher and Tornado Fang are both strong against this boss, but it's nearly impossible to hit his weak point with the erratic Ray Splasher. It fires a clawed arm at you that does a decent amount of damage to you. The machine on the top drops junk onto the conveyor belt below, which Press Disposer will come down and crush. If the junk is the top half of a busted robot, it will sitll be functional and will shoot at you. Press Disposer's weak point is the part of its head above its trunk. The trunk spits acid, but if you destroy it, acid will fill the bottom part of the screen. Still, it can be tricky to hit his head if you don't destroy the trunk. Besides, while the acid is dangerous to you, it will destroy all of the junk that falls into it immediately, meaning Press Disposer will stop moving, so it will be much easier to hit him with Tornado Fangs, which will damage him multiple times.
Whichever boss you defeat will end the stage.
Well, that depends on whether you destroyed Vile or not. This stage changes a lot, depending on that.
NEXT: Doppler Stage 2