Toho Kingdom

Rodan (Showa):
https://wikizilla.org/wiki/Rodan/Showa
Anguirus (Showa):
https://wikizilla.org/wiki/Anguirus
Mothra (Showa):
https://wikizilla.org/wiki/Mothra/Second_Generation
Kumonga (Showa):
https://wikizilla.org/wiki/Kumonga
Moguera (Showa):
https://wikizilla.org/wiki/Moguera
King Ghidorah (Showa:
https://wikizilla.org/wiki/King_Ghidorah/Showa
Mechagodzilla 2 (Showa):
https://wikizilla.org/wiki/Mechagodzilla/Showa
Hedorah (Showa):
https://wikizilla.org/wiki/Hedorah
Gigan (Showa):
https://wikizilla.org/wiki/Gigan
Takes place on Monster Island.

Verdict: I find myself leaning towards Mechagodzilla 2 or Hedorah. I remember Mechagodzilla's attacks to be very powerful, and I think it would shred through half of the combatants quickly. King Ghidorah would probably also last quite long, but would most likely ultimately die. If Hedorah is able to effect Mechagodzilla's armor, then I'd say he would win. But if not, Mechagoji will probably shred through him.

I would also lean toward one of those two. For much the same reasons.

Gigan and Ghidorah will probably run away halfway through the fight. Mogera gets blown up the second something notices it. Hedorah would probably win since no one here has Electrical attacks. MG2's Cross Attack Beam is electric but it's never really used. It's probably gonna come down to Kumunga, Rodan, Mechagodzilla and Hedorah.

Mechagodzilla, Ghidorah and Hedorah slaughter the others. Winner has to be Hedorah. No electricty means Mechagodzilla can't do anything (though he can't be melted) and Ghidorah has no way to avoid the acid grasp.

I almost wonder if it would come down to a stalemate, at least until Hedorah gains enough mass to eventually smother and crush MechaG, weeks down the road. I mean, I don't think MechaG has the means to put Hedorah down for good, but Hedorah may not have the means to put MechaG down, either. He might be able to wear the robot down with that red beam, but I'd have to imagine MechaG has the faster flight speed, and could potentially keep out of range of anything but the red beam.

Hedorah. Can't imagine anyone else having any sort of ability that would thwart his regen ability. Mechagodzilla can deal damage but he's relatively weak on the defense.

Mechagodzilla can still win if he just blasts Hedorah to peices. It won't kill him, but it will take him out of the fight.

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And I guess I passed the test yeah?



Okay...

So...
I'm going to say Kumonga & Mothra are eliminated first...
Rodan & Angurius fall next...(Ghidorah doing most of the damage)
then Ghidorah get killed in a 4 on 1 attack...
Moguera & Gigan are then blasted apart & melted...
leaving MechaGodzilla vs. Hedorah as the final two...
MechaGodzilla tries to blast Hedorah apart...only
for Hedorah to divide into smaller versions of itself
which swarm MechaGodzilla & eventually melt away
enough of the space titanium to destroy the control
mechanism inside.

Hedorah Wins!

Although I have to say that in my mind this fight would
unfold completely differently if Godzilla was present...
mainly because I believe the alliance of Godzilla, Rodan
& Angurius (& maybe Mothra?) would team up & do
some serious damage.

Although if Megalon was involved...the team of Gigan,
Ghidorah & Megalon would also be incredibly hard to
stop.

I'm done.

Theoretically it could be possible for MechaGodzilla to blow up and scatter Hedorah sludge so that by the time he reforms he either leaves such as the first time he fought Godzilla in the film or it ends up taking so long for him to reform that MechaGodzilla wins by default

I think either Mechagodzilla 2 or King Ghidorah takes this win,