Ok gang, after so many hours of fighting Larxene's AS, I finally have a strategy for you all, because I finally figured out how to activate her RC consistently. It took a long time, but I got her pattern down to where I took no damage at all. I fought her after clearing Pride Lands with the Circle of Life keyblade @ lvl 27. However, you could probably do this with Hero's Crest after clearing Port Royal just as well. Strongly recommend having Aerial Finish and Fantasia, and packing at least 2 hi-potions and at least 2 ethers.
When the fight starts, circle around her and guard her clones' charge, angling it so they don't hit D&G. When one disappears and is about to attack you, Reflect once. Most of the time she won't flinch, so close in with a couple of ground hits and knock her into the air with either Upper Slash or Finishing Leap, then continue with an aerial combo. At the end of it you should be able to use her RC. I did this several times in a row and it worked every time. HOWEVER, if she DOES flinch from Reflect, any combo will cause the RC to show, so just do a ground one. This is rare, however.
After you merge the clones, do 2 ground combos on her in a row, with only 1 finisher each. Then stop and let her attack, and Reflect it. Keep your distance: don't stand right next to her, but be close enough so that Reflect hits. If you do it right, she won't attack you with more than one lightning flash, if she attacks at all. Once it's reflected and she flinches, do the 2 ground combos again and repeat until she charges her clones at you again.
Once her clones start attacking, they'll circle you and then disappear. This time, Reflect twice as her attack is longer, then attack her and you should see her RC again. Rinse and repeat.
If both disappear at the same time, stand back and repeatedly pause to check the formation. If they surround the arena, unpause and very quickly guard. If 5 of them are at the far end, unpause and wait about 2 seconds, then guard. They may also try to do a quick slash attack on you. I didn't personally see this attack much, but spam Reflect/Guard and you should be fine.
Larxene has two desperation moves. One is where the screen blackens, then a second later her clones start slashing you, then circle and rush at you, then the real one slams down with lightning from above. Reflect, guard, and reflect again for that, then use the "two ground combo" strategy mentioned above. If she charges herself with lightning and appears in the middle of the screen instead of charging clones at you, this is her ultimate attack: if you don't have any Drive abilities, there's only one way you can avoid this, and that's with a limit. Toss a Hi-Potion on Donald if he's dead, make sure your own magic is full, then quickly activate Comet before Larxene attacks. You can't damage her until her attack is over, so ride the limit out as long as you can before actually using Comet.