(Sorry for all the words.)
I'm not that good at maths, but I can definitely agree that Alexandrite has a statistical advantage.
Looking up the calcs , that Diancie & Alexandrite get their scaling tiers from, & assuming I understand the definitions of these measurements right, the exact numbers are:
Diancie: 22.583 petatons (22,583,000,000,000,000 tons.)
Alexandrite: 20.537 exatons (20,537,000,000,000,000,000 tons.)
If my math is right, this means Alexandrite, although on the same tier, is technically about 909 times stronger. Is this a stomp? Because it seems like the numbers are REALLY not in Diancie's favor.
But, the numbers aren't everything, right? So what else is there? Well, Alexandrite has a huge advantage in melee range via the sheer size of herself & her weapons. Her Low-High Regenerationn is a massive pain, too.
So, what can Diancie do? Well, as the movie shows, it has telepathy, & can at the least, communicate with humans, so Telepathy with Alexandrite may be able to distract or confuse her, or such. Maybe? It could lean to momentary openings, demoralization of Alexandrite, or MAYBE making her fusion less stable via inducing internal conflict.... But it's questionable what dialogue between Diancie & Alexandrite could actually induce that, if they even do communicate.
Flight is a possible advantage. Diancie is a much smaller target than Malachite, & Malachite never flew. If it can block the arrows & other ranged attacks with its own attacks or projectiles, it may be able to gain some edge. Staying out of range & attacking would be a HUGE help, if Diancie can fly well enough.
One of Diancie's most useful options here is Guard Split, which would use its psychic powers to average its own & Alexandrite's defenses. Doesn't change their offensive abilities but it'd still be very helpful if Diancie thinks to use it.
The average of the 2 earlier numbers would be: 10,279,791,500,000,000,000
About 10 Quintillion. Roughly half of Alexandrite's number.
Assuming Alexandrite's AP & Durability start out the same, Guard Split getting both fighter's defenses to around here could be very helpful. Following that up with strategic use of Reflect & Light Screen to temporarily halve physical or "special" types of damage could help.... But again, assuming Alexandrite's AP starts at the same 20 Quintillion or so, she still has an enormous edge in AP.
Diamond Storm's DEF increasing would help.... Maybe Ancientpower's AP & Dura increasing, if Diancie got lucky with it.
But there's very little Diancie can do to close the gap. Sharpen to raise its own AP? But that's not much. Moonblast can lower the foe's AP with "special" attacks, a little less than 1 third of the time, but that's not much, nor often.
No clue what Skill Swap would do. Trade Clear Body for.... Regenerationn? Being a fusion?
Also, Weekly, I'm not sure what you mean by Diancie having no close-combat oriented moves besides Tackle. Besides being a small, flying target, I'm not sure why Diancie's attacks that can hit at range would be weaker at close range, other than possibly being interrupted. At the very least, it can Flail, I suppose, when low on health.
Bottom line: Diancie can fly, & has a few other useful abilities. With Guard Split, it could get itself & Alexandrite to nearly half their usual defenses.... Following up with Reflect, Light Screen, & durability boosts from stuff like Diamond Storm, Ancient Power, & lowering Alexandrite's "Special" AP with Moonblast & it could possibly tank a lot more hits from her.
The problem is, while Sharpen & Ancient Power too, can help with Diancie's mismatched AP, Alexandrite's is still over 900 times stronger before any stat changes. Guard Split, Flight, Relect & other Dura boosts would mean Alexandrite would barely hurt Diancie.... But Diancie's AP would probably be doing even less to Alexandrite.
Diancie, if it fought right, would be an incredible pain to kill. But unless it dodged & blocked projectiles & kept screens up & got lots of luck & good stats, A LOT, I don't think all those defensive advantages would actually be able to let it win the "damage race", simply because of how much & how long it would need to fight so well, against an opponent who would still hit much harder when she does land an attack.
Call me biased, but I'd vote Alexandrite, Medium-High difficulty.