You turn right around, return KH3 to the store and pick up a DS and a copy of 358/2 Days. That's how you play as Roxas.

Roxas may use his duel attack, and if Sora wins, will start using his spot beam and instant recapture attacks against Sora. Sora can use the Reversal reaction command to avoid damage, but if he fails then Roxas will knock Sora away with his Keyblade.

For Kingdom Hearts Re:Chain of Memories on the PlayStation 2, a GameFAQs message board topic titled "Roxas Card?".

Mar 29, 2007 · He does have a Reaction Command phase where you can steal his keyblades for a three-key onslaught (watch the command list and quickly select where the command appears before Roxas strikes), but this can easily do you in if you don't make the right moves, as he will light up the arena with columns of light everywhere.

Mar 29, 2007 · The Kingdom Hearts II Final Mix Ultimania (Japanese guidebook) shows that Nomura originally planned for the final boss fights to be a lot different than how they turned out: Xemnas, after absorbing power from Kingdom Hearts, was to meld into a giant Transformer-esque creature and, upon defeat, transform into a more distorted version.

For Kingdom Hearts HD 2.5 ReMIX on the PlayStation 3, a GameFAQs message board topic titled "Beating Roxas on critical".

Roxas's power to use the keyblade comes from Sora, and because Sora can use the keyblade that means Roxas can as well. When Xion replicated Roxas's connection to Sora she to became able to use the keyblade through Sora and she even replicated a copy of Sora's keyblade. What Ventus does give Roxas is the presence of a 2nd keyblade.

Riku "lost" the fight in that sense and was forced to transform to beat him. Riku wasn't in any condition to fight Roxas after Roxas sliced him with it. So Roxas did beat him yes. Riku didn't have a keyblade at that point, remember that he had to borrow Roxas' second one. He only got his Way to Dawn well into the latter half of KH2 apparently.

Roxas never needed to join back with Sora for his own benefit, it was entirely for Sora's. Roxas had unintentionally absorbed several of the memories that Sora needed to wake up and the way those memories had become tangled up inside Roxas would have made removing them without having Sora and Roxas join together impossible.

On day 358 Roxas tries to storm TCTNW with a vague plan of getting to kingdom hearts, using kingdom hearts to find sora, and then rip Xion out of sora. As you can guess this plan of course wouldn't actually work but Roxas was to sad and confused to see that at the time. Anyway he arrives in dark city and then summons both his keyblades.