A good analysis indeed Tengu. Here is an additional thought. If both teams were thrown into a room together without prior warning and told to kill (plot device to follow) I think the X-men might have a slight advantage with Professor X. He could read the minds of most or all of the JLA and communicate their powers to the X-men. This would give them a slight initivite advantage with the Prof. acting as an onfield general. I think, however, that Mimic and Phoenix tip the balance to the X-men but I could be convinced otherwise.
The League get this same advantage through J’onn, as has already been mentioned. This averages out to a slightly longer version of the ‘give the teams full dossiers’ version.
Although the League has fewer telepaths, so the X-Men could throw off the League’s efforts at telepathic coordination. I guess it’s a matter of if the Professor, Jean and Emma can jam J’onn’s broadcast before he can get the info to the troubleshooters.
Superventriloquism. Superspeed (FTL).
Of course, it will be Superman’s superventriloquism that will tip the balance and allow the JL to beat the X-Men.
Peace.
OK, both sides are forewarned that they will be TP’d (ahem, ‘teleported’) to the battle site and both told they will be facing an opposing super force that they absolutely must take out.
And so, the moment they pop in, all the heavy hitters throw their hardest, broadest attack.
A double K.O.?
What if those without broad attack powers put up their best shield?
Can the JL withstand a Phoenix/Xavier psychic blast?
Can the X-Men withstand a Fate/Zatanna magical blast?
Peace.
So far as I know, Rachael/Phoenix reappeared after Jean/Phoenix died at the hands of Magneto/Xorn. Jean regained the phoenix force a little while before magneto offed her (2 issues roughly).
If we have to choose phoenixes, I’d stick with Jean. If not, we’ll take both. 
I think we’re allowed to use both. Even with the same name, they’re different characters. Rachel’s pyrokinesis will come in very handy against Manhunter.
Who’s Xorn?
You can find him on the x-men cite i linked to in the secondary mutation thread. I missed his backstory issues, but I know he had healing powers and a miniature sun inside his head. Magneto claimed to use the identity xorn to infiltrate the X-Men, but Xorn reappeared in China with the star in his head becoming a black hole containable only by his helmet.
Since Wolverine cut the head off of Magneto/Xorn, the new Xorn is the original. Last I read he hadn’t regained consciousness though, so who knows.
As far as X-telepaths vs J’onn, it’s all X. Phoenix has power to run over the man from mars with plenty to spare leaving Xavier, Cable, and Emma free. She also has enough juice to fry any dampener thrown her way. I think Xavier could hold J’onn to a draw in a worst case scenario.
Quoth Menoccio:
This is pretty much in line with what I was seeing for that fight. Once you know what he’s got, you can handle Wolverine. So in this instance, at least, I don’t mind granting “Batman, if he’s prepared”. The thing is, though, I don’t think he would be prepared, at least not at the first meeting. While Batman has dealt with super opponents before, and has in fact even dealt with foes with powers similar to Wolverine’s, his usual run of opponents is nonpowered lunatics with weird-looking guns, and his most common tactics will be considerably less effective against someone with unbreakable bones. Given time, he’d adapt, but I don’t think he’d have enough time for that.
Of course, given a briefing in advance, it’d be a completely different story, and given a warning on the battlefield by a telepath as the battle is starting, it’d probably be a fair fight. But without advance warning, and in a fight to the death, I’m seeing three bloody holes through the armored chest of the batsuit.
Heh… no. I considered Nightcrawler - first, realize the entire X-roster is going to be disoriented form being jerked into proper position at superspeed - wondering how they covered the distance without Nightcrawler’s help. Nightcrawler might have time to get one or two people out, but J’onn will be aware of his power - he could have Wally KO Kurt before he does that. Kurt will also not be able to teleport people “on top of” J’onn - as all the X-Men will see is a swirling blur around them.
Respectfully, your father seems a bit out of touch with the teams. The full X-Roster isn’t even close to the size of the JLA; Northstar’s nowhere near that fast; and Ion would knock Phoenix cold.
I want to point out at this time that while Phoenix (Jean Grey lookalike, worried about losing control of her powers) is an X-Man, Dark Phoenix (unbridled fury) was not. She was a villainess. This is an important distinction for both sides - If abilities that a team member develops while not a member of the team counts, then Hal Jordan, Spectre, QED, thanks for comin out.
Well, fan page. Not authoritative. The intensity of Calvin’s duplicated abilities wasn’t clear in those appearances - that was back when I bought every issue of the X-Family titles. Besides, Calvin duplicates at half potency. The “quarter” reference was my hypothetical - how would the halving affect his Madrox-inspired duplicates? By halving their powers, or perhaps their numbers?
You’ll note that even that page doesn’t say he gets the ‘skills’, though.
I was counting his FTL-level speed as part of his superspeed fluctuating depending on writer, era, and editor.
Totally blanked on the Superventriloquism, though. Can’t believe I forgot that.
Incidentally : A list of JL members with Magic or Psychic abilities, as compiled by me, from the JLA/Avengers #3 cover.
Silver Sorceress - Extradimensional member of Justice League Europe, counterpart of the Scarlet Witch.
Phantom Stranger - Deus ex Machina cipher.
Sargon the Sorcerer - Master of the Gem of Life.
Despero/L-ron - Loyal robot in big bad’s psychic body.
J’onn J’onnz - Manhunter from Mars.
Maxima - Warrior-Queen of Almerac. Junior Superman class, with telepathy on the side.
Aquaman - Minor Telepath, but getting better.
Faith - Newish JLer, seems like a Jean Grey pastiche.
Dr. Fate - Well, the Justice League actually had two of the incarnations of Dr. Fate.
Manitou Raven - Current Shaman of the Justice League (now with the Elite branch)
Jason Blood/Etrigan - Yeah, the Demon and his human Sorcerer half
The Spectre (Hal Jordan) - Debatable.
Zauriel - Angel.
Bloodwynd - Formerly imitated by J’onn J’onnz.
Zatanna - Most powerful mystic of the DC Earth. No, really. That’s what they keep saying.
And… uhh…
Blue Devil.
If it’s not considered a valid source, it doesn’t matter whether or not that aspect of his powers is mentioned or not. You can’t have it both ways.
Look look look look. Folks, you’ve got it all wrong.
Yes, the JLA has godlike power and technology. But there’s one thing they don’t have - a wicked cool sound effect.
I figure it would go something like this: Nightcrawler would just teleport around, making leaps of about 0.00001 mm, alternating between forwards and backwards. While everyone in the infinitely large JLA adopts silly grins and gapes in awe at the cool wisps of brimstone and the famed ‘BAMF’ noise, the X-Men will methodically execute everyone in the crowd.
Wolverine: “Dodge this.”
(sound of silenced pistol to Hal Jordan’s skull going off, drowned out by particularly loud ‘BAMF’)
Superman: “What was that?”
Wolverine: “Oh, nothing. Hey, he’s doing it again! Look!”
Wash, rinse, repeat.
Even with lent speed, do you realistically think J’onn can read all fifty to seventy minds and come up with a complete list of who can do what and then transmit to Flash a list of those to knock out first befoe any of the X-Men can do anything? Once the telepaths are probed, they’ll be probing back hard.
So, Rogue, who can not simply be knocked out by a punch, is going to find herself in a blur that’s being deprived of air. Guess what she’ll do to the blur. And if Flash and Supes can see her coming and move, it will ruin the vacuum.
Also, there are Xers who can create area effect blasts. Dodge that. (That’s always been the weakness of Flash who can dodge directed attacks, but runs into area effect attacks).
Peace.
Also, if you figure that each team has some information on the other (at least enough to make the plan you are talking about), then team X won’t exactly wait very long to make moves of their own or to make counter moves.
Phoenix, for example, has no problem with a vacuum. A quick tk bubble on her part and the speedsters are useless. She could rearrange the ground at their feet to form a shield. I still find it very hard to believe that J’onn is anywhere close to a match for her mental abilities. She’s so far beyond Omega level mutant that it’s disgusting.
Magneto, Polaris, or Joseph could extend a bubble of magnetic force to shield them. Kurt can teleport multiple people. Kitty can phase multiple people. Rogue can teleport multiple people. Professor X could send a wide mind blast. Havok could fire off a wide blast. Cannonball could extend his blast field. Celia could extend her force field.
You’re seriously underestimating the X-Men if you think you’re three person attack will work. Their not just going to stand around and die.
And as far as Mimic goes, I’m with Aesiron. I’ve never read anything that says Mimic only absorbs anything less than full power. He fought against X-Force in the way back and Cable didn’t come come close and kept Sunspot back because of what Mimic could do with their powers. The fan site in question has done a lot of good research and are only one or two ways off in their updates (mainly roster). As far as I’m concerned, it’s good enough. They know their shizzle. 
One last point Candid that I’ve been mulling on for a while. When, in a super battle of this magnitude, has their ever been an instant defeat plan? When has any super hero fought at his utmost right off the get go. You’d never sell any books with your plan
I was teasing you. Hence the winking smiley.
Do you have any idea just how fast Wally is? Yes, J’onn could scan the entire X-Roster with half of Wally’s speed in less than a second. Not an in-depth profile, just the high points.
People with area-effect blasts that can be scanned and people who can’t be scanned should go at the center of the mass of people - so to interact with the whirlwind around them, they’ll have to blast through their own teammates.
The smarter thing for Rogue to do is fly up and out of the Whirlwind, not go through it, as it’s of unknown composition. But if she did try going through at Wally’s level, there are other options. He can grab a rock off the ground if such exists, or something one of the X-folks is carrying, and let it hit her at relativistic speeds - or, less lethally, steal her speed and leave her trapped in the whirlwind.
No information is required. With those three JLers, against a legion of random superpowered but human-looking types, vacuum generation is a quick and easy way to nonlethally subdue.
If she had the presence of mind to get the bubble up quick enough, she might be able to do it. I don’t think she would. The disorientation as the X-Forces are tossed in a pile, and then the oxygen suddenly starts to go away. I think Cable’s battlehardened enough to anticipate and save himself from such an attack - but not Jean. Remember, no one in the Marvel Universe has abilities approaching Wally or Superman’s speed. And Phoenix’s uber abilities are Telekinesis - she’s still a Telepath, but not, as far as I can tell, a more potent one than Xavier.
Okay, this is silly. “Magnetic Force”? Going to trap a supply of Ferrous Oxygen, are we? 
Kitty still needs to breathe while phased. Cannonball’s Blast Field doesn’t trap oxygen as far as I’m aware. Kurt I dealt with before. To the earlier points against him, I submit that if he began successfully teleporting anyone out of the field, Wally could just steal his speed. Havok’s blast would kill X-Men. Xavier would just be beginning to sense the presence of other minds when he blacked out - and J’onn would be only having to shield two other persons, so that’s not going to be an easy task for Chuck.
Unfortunately, the limits of Celia’s abilities are not known to me. It’s possible her force-field would keep her from a KO, forcing her to be saved to be dealt with by J’onn, psionically.
No, they’re going to pass out.
The beauty part about the three-man team is that Mimic’s powers just don’t matter in this case. Superman can only survive in a vacuum by holding his breath. In other words, preparation. I Believe J’onn is the same way. The only person on the X-Team who can survive without air is the Juggernaught, and I don’t think Calvin can duplicate Juggy’s mystic powers. Also, adding to Mimic’s disorientation - Wally and J’onn’s powers have control issues. Wally’s powers were killing him at one point; his cousin Bart’s powers were aging him uncontrollably until Wally gave him a “shock” to the system. J’onn’s powers would be particularly confounding - his years of experience and mental discipline make it look easy, but total control of the molecules in one’s body takes some practice. If Mimic has time to grab Wally’s and J’onn’s powers, he’s likely to vibrate apart into a pile of living, sentient goo.
I’m not selling books, I’m making a point.