Huh, 5 hp only? I'm not sure how i would do that, but im sure its do-able. This part is easy. Set the boss's DamageFactor to x0 for everything, and in the Pain state, add a temporary invincibility then take away 1 health, jumping to the Death state if the player's health is zero.
Also, I dunno how I would make some of the weps/the multi-wep thing work for two reasons.
One:
Survivors cant take damage.
AKA, stunning isnt possible.
Secondly:
Survivors don't lose all their weps upon becoming a boss, so I'm pretty sure that's self explanatory.
Actually, all of Spike's attacks are the same weapon. The alt-fire changes inventory flags, which affect the primary fire and the HUD display. It might be a little hard to understand, but when I send you the file it should be a bit clearer.
Ooh, I get it...it's kinda like what I did for NeoDS...more or less.
...wait, this means I can actually make Robo-Christian Brutal Sniper's rage to work properly! Oh, I'm blabbering.
Hnngwhydidntithinkofthisb4Fixed
Boss Name: Bowser
Colors: Green and Yellow
Health: 250%
Attack: Firebreath (Like in Super Mario Bros. Series)
Secondary attack: Grab Smash (He grabs the opponent and slams it on the ground with his body, like in Smash Bros., leaving half of the health)
Secondary Cooldown: 30 seconds
Third attack: None
Third Cooldown: None
Super Jump: Shell Jump (He spins inside his shell like in Smash Bros.)
Rage: Giant Bowser (He turns Giant and his attacks can deal x2 damage)
What about it now?
BTW i came with other boss...
Boss Name: Peacock
Colors: Purple and Red (But not like the Ghost of Starman)
Health: 180%
Attack: Revolver Shot (She shoots with her revolver)
Secondary attack: Bomb Throw (She throws a 8 Billiard Ball like bomb and damages the 2/5 damage for the enemy)
Secondary Cooldown: 20 seconds
Third attack: Toon Kick (She kicks an enemy with 2/5 damage that launches it very far [if hits wall causes extra damage])
Third Cooldown: 60 seconds
Super Jump: Jet Jump (She activates a Jet Rocket Fire down her tallons that brings an extra boost)
Rage: Goodfellas (She calls Andy Anvil, Tommy Ten Tons and Lonesome Lenny that hits 1/5 of the enemy and they leave, then a little bomb comes and Peacock can toss a cigar, if the cigar hits the bomb, it explodes and the explosion can deal 1HKO.)
You want 250 hp for bowser, and 180 hp for Peacock I assume.
Well, for one, the alts arn't do-able because:
Survivors cant take damage.
I've said that a lot. lol.
Anywho, the rage for peacock also has ^ that same rule. Bowser's on the other hand, I'm sure is do-able.
Primary's and Super Jump's look good though. Nothing I want to change there.
EDIT: oops, forgot about this:
I do technically have two ideas, but since one needs a custom skin and likely won't be accepted anyway (I'm really deep into my own fanon now), I'll only submit the one for now.
Boss Name: Quote
Our hero from Mimiga Island took one step too close to the edge. After plummeting to the surface, he found the world of Mega Man beneath him. Mistaking the Robots for those who wanted to seize the Demon Crown as their own, he begins to exact his own vengeance...
Skin: MM-Style Quote (I know there's one around here
somewhere...)

(I know it exists, I just don't know where.)
Boss Theme:
(Try the Cave Story music pack!)
Colors: Assuming it goes Blue/Cyan... Black/Red.
Health: 150 Per Player (Not sure what's balanced...)
Attack: King's Sword. Quote slashes in front of him with King's powerful sword.
Secondary attack: Missile Launcher. Quote fires a single missile from his Missile Launcher. The projectile moves quickly and has a rather large explosion radius, but it does pretty light damage unless you get hit with the missile itself.
Secondary Cooldown: 60 Seconds
Third attack: N/A
Third Cooldown: N/A
Super Jump: Quote propels himself straight up by using his Booster v2.0.
Rage: For 20 seconds, Quote begins to dual-wield his two best weapons: The Spur and the Nemesis. The Nemesis is one of the highest projectile speeds and is a 1-hit-kill, but the more you use it, the worse it becomes--after a kill, the Nemesis becomes a 2-hit-kill and travels half as fast. After three kills, the Nemesis becomes the worthless duck launcher we all know and love. The Spur has speedy fire rate and does decent damage, but its true danger lies in its charged shot--holding down the mouse button for a time and releasing will fire a massive ripper wave that's almost certain to kill whatever it hits.
Well, firstly I might just make that alt a 1hko if you get near it bec--you know what. I've said that enough.
Secondly, The rage...I'm not sure if I can do what SmashBros said about his alt, for the twin weps...wait, nvm I think I can do that.
BUUUUT, survivors ca--*shot*
meening, the nemesis cant have that effect. hnng.
For his other wep, I would make it so it fires a normal 1HKO shot IF fired normally, no ripping. If charged, well, the massive ripper thing you said sums that up.