I figure they're about even in telepathy with perhaps a slight edge to John, not sure. But even so count in all his other powers and then let Emma have her diamond form with her telepathy. I'd say its pretty damned even.
John's got lots of powers but in her diamond form I don't see how they can hurt her and vise versa.
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J'onn stomps her, quite frankly. The thing "on our Earth" kinda balances out things reducing the very "TP distorting" conditions of MU, but not enough to make Ems a formidable opponent there
The thing that can kinda help Ems in the fight is that astral plane duels aren't well-practiced by DC psychics, so one surprise win is guaranteed, but other than that Ems gets BFRd\overpowered 9\10.
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*Remembers experimenting with Ems\Miss Martian amalgam in the tourney and getting 4th overall powerful char of it
Gender: Male Location: Drinking gasoline. Pissing napalm.
Sure, I've got that down, too. So, we can pretty safely agree no one is getting mindraped in this fight, etc.
Emma's diamond form makes her somewhere around...class 50, strength-wise. Something of the sort. Whereas J'onn is nearly on par with Superman strength-wise. Massive advantage there. Ditto for speed. He's also got the intangibility thing going on and the ability to change shape.
TP, I'd say they're close enough to not make a difference. Physically, he stomps her.
The entire combat is going to be telepathic, not physical. And what I've described is a results of TP confrontation between the two (J'onn has eons of experience, Emma compensates for that by being great at improvisations) where Emma can get quite a few wins for sure...But most close thing is stalemate.
Physically, J'onn can damage her in some ways whic would be OOC for him, though, but diamond form is pretty much her saving grace against brute force. Physically she does nothing to J'onn (not strong and versatile enough) while he can at least BFR her.
Class 50 is generous. Emma isn't even class 50 IMO.
She's hella durable, but has she ever been shown to actually do anything physically impressive? I'd rate her in the 2-10 ton range.
She can most likely simulate class 50 hitting force due to sheer hardness of her body, but as far as lifting stuff goes, she's class 10 at best, probably class 5 realistically. She's not a heavy lifter.
As far as the fight goes, it's either boring TP clinch\stalemate, or Martian Manhunter BFRs Emma (can't break her body without OOC things with phasing) and wins that way.