She's already defeated me three times...like right now! I've never even tried to master sleights and in this case Larxene is way too diddlying fast and long ranged with her attacks for me to get the cards arranged. One defeat was especially frustrating and one of those scenarios where I really get pissed off at the card system because she wouldn't let me go to reload my diddlying cards.
In a manner of speaking, one of the tricks to the game is finding a way to win while reloading cards as rarely as possible, usually by setting up your deck for the best possible sleights. At the
end of the game, even using the third reload probably means something has gone wrong - though stuff goes wrong all the time so maybe that's understandable. The trouble is, this isn't the end of the game. Larxene hits you exactly at the wrong point for deck building strategy. She's hard enough to warrant it, but you haven't had many opportunities to get cards, you only have so many sleights, your CP is probably low to begin with... oh, and you have no Elixirs or Mega-level items because they aren't introduced until later. kuraudo is right: Sonic Blade is the best way to win, but let's see what we can think up by reinforcing a few fundamentals.
Do you have a crop of 0 cards in your deck? It's only half-way through the game so you very well might not, and 0s are expensive in terms of CP (once you're not scraping to even find them, it's best to just use 0 Keyblade cards since they cost less and take the least time), but it's critical to the game's strategy to have them on hand next to your reload card. Some folks like them at the end of the deck, I like them at the front, even if you have to flip past them at the start of the fight, because then they're only ever one button press away should the boss do a Sleight. Whichever side they're on, they'll definitely be there while you're trying to reload, which is an added bonus. Remember you can break enemy attacks with 0s or strong cards even if they're nowhere nearby. Now, Larxene doesn't always use Sleights to break up your reloading, which is part of what makes her a pain, but one step at a time.
Later in the game you'll probably want some 0s in all your decks but don't forget you can save three decks in your save file, it helps to make one a special deck to use against Sleight-wielding bosses, and a second for generic Heartless. (You could probably save the third for non-Sleight using bosses or something unusual like White Mushrooms but I just... don't. I use it for stupid experiments that never amount to anything and I refuse to learn.)
Definitely chug Potions after your second reload instead of going for the third (or build up toward the third but keep relying on Potions instead when things get hot). Standard Potions aren't worth it compared to Hi-Potions, unless you just aren't using Sleights (Hi-Potions restore cards lost to Sleights. And you really should use Sleights if you can, sometimes I'd even say to use them even if they're just random assortments of cards). If Larxene's breaking your attempts to use the item, perhaps because your item card stinks (you get a powerful 1 card later in the game, the thing is junk), I like to cram in a few random Keyblade cards after each item and use them as a Sleight with the item at the front. So long as the Sleight totals 10 or higher, you'll trump her normal attacks, and you only have to last long enough for Sora to down the item, which is all of three seconds. If she breaks the combo after that, who cares?
Since Larxene isn't weak against any spells, it might be best to ditch Spells from your deck in favour of more 0s or keyblade cards you can use to set up Sonic Blade: this also lets you ditch your Ethers, unless you've got enough Cure cards to justify keeping them. I really don't like carrying Potions
and Ethers because they bulldoze their way through your CP (Elixirs are just as bad, but by the tiem you get them, you'll need them). If you have any awful Cure cards, stick them at the front of the others and use them as the disposable cards in Sleights, you'll get more healing that way (in fact, that's a good idea in general). See if that helps you cope without the Ether, it'll probably be worth the cost of CP to try.
Remember that if you can free up enough CP, you can also use Oogie Boogie's Enemy Card to gain Regeneration for a time, which is a game saver I really abused my first time through the game. This is a great antidote to Larxene's interruptions. She hit you? You'll heal it without trying! Run away, keep reloading. It's not perfect but it just might pull you through.
That's really all I can say without going into advanced deck-building strategy, aka breaking the game in your favour. Someday! The best I can add is that if you must grind, consider grabbing any Bounty rooms you missed (one unique prize a floor), visit Moogle Shops and pray for 0s, Cures and Hi-Potions, and to always level up Sleights and CP until you can build the deck you need or until you're screaming for more HP (which you might be, at this point). In fact, do that all the time. Hope something helps. Good luck!
