First off, sorry for making such a wordy post.
Anyway, question: Possibly something for a revision, but... Do Pokemon, like, in this case, Alakazam, get all the abilities of their pre-evolutions? For example, would Alakazam have all the abilities of Abra & Kadabra that their Pokedex entries mention? If so, Alakazam has a bunch of other options.
There's also the matter of its Durability & Stamina that I wonder about. Our profile claims its durability -power-scaling aside- comes from Psychic powers that are always active... but then again, Alakazam's Pearl entry claims it can use all forms of Psychic abilities, so whatever. Although there is also:
| Ruby | Alakazam's brain continually grows, making its head far too heavy to support with its neck. This Pokémon holds its head up using its psychokinetic power instead. |
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| Emerald | While it has strong psychic abilities and high intelligence, an Alakazam's muscles are very weak. It uses psychic power to move its body. |
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Of course, this could merely be a matter of stamina, & may not matter as much to Bambietta with her Reishi, The Explode.
Anyway, onto Mega Alakazam. (Which, incidentally, has Pokedex entries also supporting physical frailty, or weakness, even if it is apparently protected by constant psychic powers.)
| Su | As a result of Mega Evolution, its power has been entirely converted into psychic energy, and it has lost all strength in its muscles. |
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| Moo | Its hidden psychic power has been unleashed. A glance at someone gives it knowledge of the course of that person's life, from birth to death. |
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If Mega Alakazam has all Alakazam's abilities, too, this may also be worth considering:
| Su | It is said to have an IQ of approximately 5,000. Its overflowing psychokinetic powers cause headaches to anyone nearby. |
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These headaches may also be very intense, if they are present with Mega Alakazam; Not only does Alakazam have them, but so does Kadabra. Although, if they're a product of OVERFLOWING psychokinetic powers, they may not be a factor with Mega Alakazam.
With Mega Alakazam's telepathy & hax to find out about her powers from the get go, & its supposed smarts, I'd assume it would immediately be able to learn about her abilities, either via telepathy, or... just taking a glance.
The Explode is dangerous, with its durability negation, turning things into bombs, & being a form of omnidirectional attack.
As for combatting The Explode, Mega Alakazam has teleportation, telekinesis, & Disable's power nullification. 1 tactic I can think of would be, should Bambietta use The Explode, to use Disable to stop her using it again.
Should that fail to stop it, since they [The Explode] are "Reishi that turn anything they make contact with into a bomb itself.", would be to telekinetically rip apart some part of the surrounding environment, or whatever materials are handy, & send them to The Explode, triggering the Reishi without MA itself touching The Explode.
From there, it could teleport to safety; At the very least, I assume because Bambietta's profile claims her range is only "Hundreds of meters", while Alakazam's is "Several kilometers".
With all this, her Reishi may not prove very effective. Nevermind the possibile option of telekinetic redirection.
MA can deal with otherwise problematic close combat via telekinesis, precognition, teleportation, Regenerationn with Recover, & Reflect to halve physical damage.
Alakazam may not like physical damage, & while she has means to buff her physical offenses & defenses, Alakazam likely already knows about them by the time she uses them, & has a number of options against her. Alakazam can also stat amp with Calm Mind, likely inflict Confusion with Confusion (Duh) & Psybeam. Bambietta may also have to deal with headaches, too.
Overall, I don't know Bambietta THAT WELL, & she likely has an advantage, offensively & defensively, Mega Alakazam can combat that with Calm Mind, Reflect & Recover, & use Disable on problematic techniques.
Mega Alakazam also has telekinesis, teleportation, telepathy, precognition, block The Explode by using surrounding materials while dodging, can inflict confusion on her with its attacks, & likely much more.
Not to mention it probably knows her quite well from the get go & she may have to deal with headaches just for getting too close, which she may need to do to get an edge in via physical combat.
Bambietta's flight may give her a mobility advantage compared to MA's mere Levitation, but with Telekinesis & Teleportation, it can likely fight that well, too.
I vote Mega Alakazam, Medium difficulty.