So Ive been playing the orginal chain of memories on gba and I got up to where you have o fight riku rwplica after clearing Destiny Islands. I must say that it is so FIRETRUCKING impossible. Im level 49 have all the level up sleights i have 775 cp and uave just breezed through the rest of the game. But this is RIDICULOUS, he breaks EVERYTHING, and the one where he teleports everywhere is so annoying, I keep running out of zeroes, I have 6

 

So any help/tips?

From the Kh wiki:

 

When fighting the Riku Replica, you need a few enemy cards: Jafar, Oogie Boogie, Vexen and Card Soldier (to name a few). You will also need a few Magic Cards, Attack Cards (numbered 7-0) and Item Cards other than Potion. Equip the Jafar enemy card and start attacking. When that runs out equip the Card Soldier enemy card and use that, then when those run out use magic and other cards but save the Oogie Boogie boss card until you are low on health. Another option is to start with the Vexen card and, after you get killed, use the Oogie Boogie card or a bunch of high-numbered Cure Cards, then use the Jafar or Card Soldier Card.

 

Without boss cards, you should stock up cards numbered 7-9, an Elixir and a Hi-Potion at the least, two or three Stop cards (for the Lethal Frame sleight), two or three Cure cards and some 0 cards at the end of the deck (to break the Replica's attacks). You might want to insert some Premium Cards in to save CP.

 

Use sleights when possible; sleights are an effective way to cause great damage to the Replica (depending on which sleights are used). Keep bashing the Replica with Sleights, reload (using an Item Card) and heal when necessary.

 

As a final alternative, you will need the Sleight Sonic Blade and different types of attack cards with high numbers to form Sonic Blade. You will also need to put the cards in order to keep stocking cards. At the end of you deck you should put an Elixir or one or two Hi-Potions. A Cure card is optional. If you don't have many cards that can make Sonic Blade, use Blitz. It is also good to have Cloud cards in the deck, though this is optional.

 

Stock up three cards to get the Sleight Sonic Blade and use it. Try to get all the hits in, then keep doing this until you run out of attack cards. Use an Elixir or Hi-Potion to recover all your cards. Keep repeating this to easily beat him.

I just spam the sleights I have. Ars Arcanum will end the battle rather quickly. 

You're a high enough level. Just keep a cure card on your hotslot for whenever your hp goes down and make sure to use sleights for less card breaks.