This is not an easy match. I'll say in advance that I'd place my bets on Mirage Mewtwo, with High Difficulty....
....But the difficulty is in that MM has to use out-of-the-box tactics, & avoid a straightforward fight; Accelerator passively redirects vectors, & it has been argued vector redirection is independent of AP. His intelligence is also stated to be better than 100 supercomputers.
That means that even if MM copied his Vector Manipulation with the Trace Ability, or Mimic or Sketch Moves, & employed Metagross's &/or Alakazam's superior-to-a-supercomputer intelligence, it still wouldn't have a good shot, because any vector attacks would be redirected, while Accelerator freely attacks MM; Accelerator's own attacks & many of MM's own moves hitting MM.
Even if there were somehow 100 Mirage Mewtwo who'd all copied Vector Manipulation, it STILL might not have the edge in a fair fight; Again, Accelerator's intelligence is superior to over 100 supercomputers & even if MM did get that many duplicates, it isn't a sure shot they'd have the co-ordination to work together to manage as many or more Vectors as Accelerator &/or do it better.
Never mind that depends on the Mirage System BEING ABLE to simulate 100 or so (More or less) Mirage Mewtwo all using most or all of their own Supercomputer+ Intelligence.
In a straightforward fight, Mirage Mewtwo is SCREWED, simply because Accelerator can manage vectors better than it can, severely limiting what moves MM can use without risk of Accelerator turning them on it, even if MM had a considerable numbers advantage & all of them were using the most of their intelligence.
Googling tells me human eyes can see up to 30 miles, or 48 KM.
So why am I still on Mirage Mewtwo's side, you might ask? Well, the good news for MM is, IT PROBABLY KNOWS THIS, TOO. It has Xatu's ability to see the Past, Present & Future all at once, Abra's ability to detect foe's & hostility, the aforementioned superintelligence, & probably a bunch of Enhanced Senses from other Pokemon that I'm forgetting, even using only Pre-Gen 4 Pokemon.
Mirage Mewtwo almost certainly knows Accelerator is coming & hostile, possibly even in advance, likely before Accelerator knows it's there, & given that it can read his mind, see the past, present & future, it probably knows it doesn't want to fight Accelerator in the usual way, should know it needs a plan, & thankfully, can probably find time to make a plan.
So, with the abilities of nearly 400 Pokemon, how do you take on somebody that can thwart almost anything with a basis in physics & even teleportation?
Before we get to that, there's one other big question: Do we consider MM dependent on the Mirage System to function? Can it leave the system's range? Is Accelerator willing to/allowed to destroy the system?
If MM depends on the system, there's more difficulty, because it has to make sure Accelerator doesn't destroy it. Not easy in a battle of 2 At Least High 6-As.
Ideally, it'd be able to teleport or otherwise relocate the Mirage System before Accelerator gets too close, & hide it underground, assuming its mind reading tells him he isn't aware of it. Otherwise, it may be less telling to leave it where it is. Besides that, relocating it requires that the Mirage System CAN still function -ex: Possible power supply issues?- after being moved.
1. Is Time Travel (Via Celebi's powers?) possible for Mirage Mewtwo, given its dependency on the Mirage System? If it is possible, Time Travel to a time before the fight, when Accelerator is asleep or comatose. Employ Mew's ability to become invisible at will if stealth is needed. Assault Accelerator's mind with Dream Eater. Since it's Mind/Dream Manipulation, it should be able to bypass Accelerator's Vectors. Hypnosis can help keep him asleep. If Time Travel doesn't work, Hypnosis+Dream Eater may still be viable if MM has to directly face Accelerator.
2. As mentioned, MM has Mew's Invisibility at will. It starts out of range of human sight. If Accelerator isn't aware of it, it's time to try not even facing it! Lots of Transform use to follow. Supposing Jirachi can't grant wishes to itself....
First, hide. Easiest is underground & invisible. Get container, using heat to make glass from the ground if necessary. Shape into container, use Water or Ice to speed hardening up. Turn into Skarmory &/or other bird Pokemon. Shed metal feather. Use alone or with other bird Pokemon feather to make a quill, with Skarmory feather as a metal covering for it. Fill container with Smeargle ink. Turn into Muk/Grimer. Duplicate a Grimer. Get Grimer to obey temporarily, Hypnosis/Mind Control if necessary. Turn into Jirachi. Get the new Grimer to write on the Jirachi tanabata charm thing, a wish that Accelerator never existed/doesn't exist anymore, etcetera.
If this worked, congrats, Mirage Mewtwo, you've wiped Accelerator from existence by abusing wish granting powers! If more time was needed, Hypnosis, Encore & more methods can buy time in addition to hiding! If this didn't work? NEXT PLAN.
3. The main reason Accelerator is a threat in a straightforward fight is because, again, Vector Manipulation means he can turn most options against their user. But what if he couldn't? Accelerator uses Vector Manipulation.... Mirage Mewtwo used Disable! If this disables his active use, that's okay, but his passive VM is still a pest, but at least Accelerator has far less offensive presence.
If this Disables his passive VM, hooray, most of his defenses are down! Employ Hypnosis if set-up time is needed, set up stat amping, preferably with Calm Mind (Mind/Dream Manipulation is still safer than physical attacks.), or power up via Alakazam's power to use all its abilities to their extremes by closing its eyes. Start assaulting the less well defended Accelerator with Psychic.
If it Disables both his passive AND active VM, perfect! Proceed to set up &/or attack.
But what if Disable isn't enough? Well, it can lock Accelerator into a less harmful action, or maybe into VM use it can work around with Encore. Or it can, in a disturbing possibility... turn Female & use Attract on Accelerator to Infatuate him so that Accelerator takes less action & maintains less Vector control.
Teeter Dance can also leave Accelerator confused, & if MM can switch abilities, it can go to Own Tempo to not get Confused itself. More opportunities!
4. Somehow all those options won't work either! Well, MM can mix & match abilities & moves, right? As early as possible, before Accelerator is too alert/hostile, get in maximum earshot range & invisible.
Change ability to Soundproof, use Perish Song; Soundproof blocks Perish Song so it doesn't affect MM, but does affect Accelerator. SBA says to assume the characters don't know about each other unless their abilities support it, & Accelerator, not knowing the song, may be curious what the heck is going on/what in the world MM is doing singing. Unless he's moving too fast for sound to reach him or blocks the song's vectors -He has little motivation to do so without knowing what it is, though- it should affect him.
If PS hit, proceed to defense, & wait for THAT to take out Accelerator. Go on defensive so he doesn't kill. Thankfully, Wish can be reliable healing -as Accelator's VM can impair biology based healing- & intangibility -such as via being Ghost-type- can help prevent Accelerator messing up healing. Pain Split may also help, as would Cosmic Power.
5. Accelerator won't hear Perish Song! Now what? Mirage Mewtwo can change Type, right? Turn Ghost-Type, use Ghost-Type Curse, fight defensively until Curse kills him.
6. Try & win in a straightforward fight anyway. It's.... not QUITE true that MM is screwed in a straightforward fight here. In a damage race where it's unprepared for VM, probably, but that MIGHT not be the case here.
VM severely limits MM's offensive options, but they don't disable it, & MM lacks a Mind for Accelerator to manipulate. I don't know if he can manipulate its body, as its a simulation, but in a fight, Mirage Mewtwo can fall back on Psychic to assault the mind, Hypnosis or Yawn to put Accelerator to sleep, Dream Eater to attack through Dream Manipulation, possibly boost its own stats with Growth, Amnesia, Calm Mind & Cosmic Power & more, impair Accelerator with Teeter Dance, Encore, Disable, Attract (If proper gender & possibly proper species.) & has multiple choices for healing, several forms of Precognition & Mind Reading.... Stamina reduction through Pressure & Spite, probably Duplication through Muk & Grimer's abilities.
It can even copy Vector Manipulation with Trace, Sketch or Mimic, even if Accelerator's intelligence probably means it can use THOSE much better.
But when you consider all the abilities it has, it probably has a lot more staying power. Protect also helps, & many ways to impair him, even if it needs to protect the Mirage System. At the least, it may even be able to force a draw via Destiny Bond.
Again, Mirage Mewtwo, High Difficulty. It has to work out of the box, but its abilities let it be well prepared, plan & work around Accelerator's key abilities.