Mega Man Xtreme/Wheel Gator

Arm Upgrade / Zero Rising
After the yellow "bird nests" and two ladders going down, you should jump down to a lower screen. Instead, if you climb up the wall, you can see Dr. Light's capsule. You need the Leg Upgrade to reach it, and you'll get the Arm Upgrade. In Easy & Extreme Modes you'll obtain the Zero Scramble Rising instead.


 * "Install the program Zero Scramble Rising! This will allow you to call your best friend and fellow soldier, Zero... His striking power is enormous, but hard to control! Perform this on enemies in the sky. It also works on bosses!"

Ride Armor


Continue until you find the Ride Armor in plain view: it is necessary to destroy the wall in front of you and defeat a series of tough enemies.

Heart
The Heart is visible above a spiked wall. Use the newfound Arm Upgrade to charge up the Rolling Shield, and you can walk on spikes!

Maverick: Geemel


Geemel is an absolutely optional boss. If you want to accept this challenge, take the red lift to the very top, instead of getting off halfway to the left. Grab the spare body, then enter the door on the right. Fight him using the X-Buster: he is not weak against any of the weapons in Hard Mode.

In Easy & Extreme Modes you can use the Speed Burner to defeat him faster.

Energy capsule
Leave the room and select the Silk Shot: it's the strongest weapon in your possess now. Continue advancing to the right until you get at the end of the screen: an energy capsule is hidden behind a grey construct.

Maverick: Wheel Gator


Wheel Gator is weak against the Magnet Mine. In Easy & Extreme Modes, you can also use the Zero Scramble Final.

Attacks:
 * Spin Wheel
 * Mouth cannon
 * Jaws from below
 * Screwdriver attack

The weapon obtained from Wheel Gator is the Spin Wheel. It is necessary to reach the fourth capsule of Dr. Light, but also it's the only weapon from the cyberworld effective against the real world Sigma.

Body Upgrade / Zero Earth Geyser


Before entering the Computer Core, return to Morph Moth's stage. Right after the first save point, and just under the very first step, there are tiles that can be destroyed by the Spin Wheel. Jump down in the hole and reach Dr. Light's capsule. He'll give you the Body Upgrade in Hard Mode, or the Zero Scramble Earth Geyser in Easy/Extreme Modes.


 * "Install the program Zero Scramble Earth Geyser! This will allow you to call your best friend, Zero... You can damage enemies around you! This requires a lot of energy... Use this only in a desperate situation."


 * Errata: The game says "Gaizer", but it's actually a misspelling for "Geyser".