Spike Man: Spike Man's stage takes place in an abandoned city at daytime. It's full of spikes and conveyor belts (most of which move forward, unlike his big bro, Metal Man). Speedy travel combined with spikes can make this stage a tad hectic.
Spike Man's boss room has spikes on the ceiling and under the floor. Spike Man can pound the floor to cause some of the tiles to go up. When he does this, stop (if you continue to run, you risk running into a spike ball; if you jump, the added momentum may send you into the ceiling). He also fires spikes. He alternates between two diagonal needles and one straight one. Additionally, Spike Man can fire them as he pounds the ground, so be careful. His weakness is the Boulder Toss.
Spike Man gives you Cross Spikes, which allows you to shoot off two diagonal needles. Although it's an awkward weapon to use, it packs a lot of power, especially against Ghost Man or Zero.
Boulder Man: Boulder Man's stage takes place on a mountain. Most of the first half is spent riding Rush through the air in a Gradius-style shooting minigame inspired by Tengu Man's stage from MM8. The second half is spent on solid ground, dealing with the likes of Crag Men and other such enemies.
Boulder Man has two life bars (ala HS.H) and starts out as a giant Crag Man, throwing boulders at you. He can also turn into a giant boulder and roll around the room, shaking it whenever he hits a wall. After taking down the first half of his health, he'll reveal his true form of a normal-sized Crag Man. He throws rocks the same way his bretheren do, but he follows the traditional RM rules of 28hp and momentary invincibility. He's weak to the Flood Nozzle.
From Boulder Man springs the Boulder Toss, which is similar to the Super Arm, except that you don't need an existing boulder to use it. Use it on Spike Man.
And that just leaves Neon Man. Any questions?