Glad you took down everyone's favorite dude who is "not just any old dude" without any problem.
He was ironically one of the easier ones for me due to how surprisingly straightforward his battle was.
Don't worry, man. The command meld system is a bit of a pain to get used to and it seems overwhelming, but believe me when I say that it can be extremely rewarding.
hemmoheikkinen essentially said everything that you needed to know. While Once More and Second Chance may seem like a pain to get and level up, they're definitely worth it for the battle as well as for the game long term.
Marluxia's Absent Silhouette is bar none the most difficult of the five. All I can really say is that you must guard strategically (and remember to do so the instant the battle starts) and use Reflect whenever needed. Don't worry about health too much since most attacks will 1 hit KO you unless you have Second Chance and Once More (I picked the sword, so I didn't at the time). Also, by guarding after he summons the rose petals and attacks with a flurry, you can either get a Reaction Command that restores your life counter or one that deals damage depending on where you are facing when it appears. Ironically, I consider his Data Battle to be the easiest.
For Larxene, all I can really recommend is to constantly spam Reflect when in a bind. Limit Form helps quite a bit as well.
Zexion's battle is actually fairly easy so long as you can avoid getting hit by his meteors and have Reflect on your side. Just attack him relentlessly once your warped back to the islands and you should be fine.
Vexen is a pain. Reflect when he uses that one ice move that freezes you and requires you to mash triangle to get out. Also,
take out his Anti-Sora clones as soon as they appear for the Absent Silhouette battle. Use liberal use of Reflect and Fire magic to take him down.
Lexeaus is admittedly one where I needed Peter Pan to help me the first time around. That being said, he only helps for so much. Just guard where necessary so that you can use the Reaction Command to get some of his increased power. Also, Dodge Roll helps.
Awesome! Glad to hear you took him down.
I'm completely with you here. Terra's Lingering Sentiment was a severe pain in the neck! I'd love the chance to troll him after the crap he gave me as well.