Mega Man X8 (16-bit demake) The secret Boss trinity.

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So here we have the 3 Secret Bosses that the game has to offer.
Cut Man was also in the original, but an absolute Pushover, usually defeated by one charged shot. You need to clear all challenge rooms with the best score.

Cut Man (Mega Man 1) is unlocked by getting an S Rank in all 3 Challenge Rooms in Troia Base during a single Run.

Serenade (Mega Man Battle Network 3) is unlocked by having 100% Completion on all Maverick Stages.

Red (Mega Man X7) is unlocked by entering the Secret Boss Area without Armor as well as deactivated Sub-Tanks and Heart Container.

Cut Man is very easy. Just don't facetank everything and you're good. He's 1/10 Difficulty

Serenade has a mechanic that persist over the fight. He has 2 counter tokens . Consume them by hitting him, but you need to dodge either by precise jumping or shadow dashing. The counters are very fast and hit hard.
Key is to count the times he teleports. He starts an attack after he teleport's for a fourth time. So hit him once during the first 2 teleports and once more before the fourth. He's then vulnerable to attack while he's attacking.

During phase 2, Kanji's will appear from time to time which covers most of the screen, and when you have the choice to attack or dodging it, i would recommend to dodge, as the Kanji deals the most damage by a lot in this fight (Half of your health if not more). Dodge them by standing in the area's that are not covered in color.
12 attempts were needed to beat him the first time. Difficulty 8/10

Red too has a mechanic that determines the fight. He deals only 1 damage with any attack and contact damage, but he'll recover health whenever he hits you (2 health), and some of his attacks are extremely nasty to avoid.
It took me 4 attempts to beat Red, and that was actually a really clutch one.
Difficulty 6/10