Kingdom Hearts Chain of Memories (review here) has some of the most convoluted, but incredibly deep combat in the entire series. In fact, the combat is so convoluted, that I can't even fully explain it in my review. That's why I wrote this, I wanted everyone to not have to go through the incredibly painful process of not understanding what the heck is going on. This is a comprehensive look at the combat, leveling system and deck construction in the game. So it's gonna be a little long.
Basic Combat Structure
Battles revolve around cards. Here's a picture of your typical battle. Top left, health, bottom left, deck, bottom middle, the cards currently in play. Sora just stacked three cards together and is casting Thundaga. The deck on the left is a cycle of your unused cards. Cards are played left to right starting with the card in the position of the "one" card, so you can see two cards about to be played and one from the previous part of the deck. The solid black card with the number in the middle is the reload card. When you run out of cards, you charge the reload card in order to restore your used cards. Charging the reload card leaves you vulnerable because you can't move while charging it. Each time you use the reload card, you have to charge it for a longer period of time. The number on the reload card increases each time you use it up to the maximum number of three.
In the image above, you can see several cards in play. Each card has a color, number and picture. The color tells you whether its an item or summon (green), attack (red), or magic/summon (blue) card. Cards are played in real-time by both you and your enemies in real time. Playing a card starts an attack and that card stays on screen until the attack animation has finished. During battle, only one card can be played at a time, whether that's your card or the enemies card. The number on the card determines priority during battle. Numbers range from 0 to 9. The higher the number, the higher the priority for the attack and the more cards that you'll be able to beat in order to execute your attack card. If your card is higher than your enemies card, you attack unless you get interrupted by a higher card. In the same way, you can interrupt enemy attacks by using a higher value card than them. In this way, depending on how you've constructed your deck, you can almost entirely prevent an enemy from attacking by using high cards, and dodging to avoid attacks. The exception to this rule of priorities is the zero card. The zero card cancels any card, but since it has the lowest value, it can also be broken by anything. All of these rules work in boss battles too, so while attacks themselves are harder to dodge than in KH1 due to a smaller 3-D environment, you can cancel every single attack they throw at you with a well constructed deck.
Combos and Sleights
Cards can also be stacked up and played together to combine the total values and actions of the cards. You can play up to three cards together, with their values combined as a composite value; executing all three attacks consecutively. Additionally, certain combinations of these cards turn into special attacks called sleights with special status affects and attacks. Sleights usually create special combo attacks: Strike raid, Sonic Blade, Trinity Limit, Holy, but also execute more powerful versions of magic attacks, summons to a grand total of close to 90 other completely unique combinations. Some sleights require three entirely different, very specific cards. Some just require that the total of the cards lies within a certain range(15-20) and are a certain type (red cards). Regardless of which cards they use, almost all of these sleights are useful, and it's your job to construct the best possible deck using them. The problem with using sleights is that it consumes the first card of the combo semi-permanently. So the next time that you reload the deck, you can't use that particular card. You can either build multiple complex layers of sleights into your deck to prevent this from being a problem, or you can reload using special item cards. Here's a list of sleights: http://www.khwiki.com/Sleight
Attack Cards are found as you progress through the story, as you beat worlds and in chest rooms. Each card has a damage rating, a recovery rating and a status effect. Damage rating obviously refers to how much damage the card does when it is played. A Kingdom Key, the weakest card, has a D+ rating. Ultima Weapon, has an A+ rating. All the other cards go anywhere else in between. Keep in mind the 0-9 itself doesn't affect the damage output, the rating is what decides that. A "9" Kingdom Key will always be less powerful than 3 "Wishing Star". But having the weaker Kingdom Key in your deck might be a good idea, because a card is no good unless it can beat the enemy cards in play. Recovery rating refers to how quickly Sora can play another card after his was broken by an enemy card. The higher the rating is, the sooner the animation and hit stun end and the faster he can roll, jump or attack. Finally, some kind of unique attribute about the weapon and it's use. Metal Chocobo — "A special attack card that can break through physical defenses. A bit difficult to handle." vs. Three Wishes — "Fairly strong with a fast swing." You get my gist?
Summon Cards are unlocked as you progress through the story and beat mini-bosses. Some of them can be found in bounty chests and "rooms to rewards", so make sure that you open at least one treasure room in each world for a unique card as well as revisit them with your room of rewards card. Notable summons include Cloud, Genie, Tinker Bell and Simba. These usually do something unique like stun enemies, enter aiming mode to shoot fireballs, or just make Cloud appear and start killing things. I only ever used Simba and Cloud, but depending on what you like any of them could be useful. Some really cool sleights like Mega-Flare, Reflect Raid and Omni-Slash use summons, so don't get rid of them.
Magic Cards are your basic magic attacks from standard Kingdom Hearts/Final Fantasy fare. Fire, Blizzard, Aero, Thunder, Cure, etc. Stack up to 3 together for a more powerful version as usual. What's best about these cards are their use in strike raid variant sleights. Judgement, Thunder Raid, Fire Raid, etc. are all awesome. There some other really cool sleights, so don't get rid of these either.
Item Cards include potions, ether and elixirs. Depending on how you build your deck, these can be really useful, or really useless. Item cards provide instant reloads for your deck and in some cases can even restore cards lost during sleights/premium cards. Instead of waiting to charge your deck, losing any cards from sleights, and increasing the charge counter each time you use it, using an item card just refreshes your deck immediately. Potions and Ether reload attack and magic cards respectively. Hi-Potions, restores all of your attack cards including lost cards. Mega-Potions, Mega-Ethers, and Megalixirs restore all attack, magic or both cards respectively and reset your charge counter. If you use a lot of sleights, these cards are majorly useful. In my final deck, I used weak megalixirs because they're so rare and hard to find, but then I combined them with some extra, high numbered attack cards to make sure they don't get card broken by enemies. Some people like to combine Mega-Potions and Mega-Ethers to do the same thing as a Mega-lixir. However you approach it, and whether you choose how to use them at all it's obvious how useful they can be. Item cards are consumable per battle. So if you have two potions in your deck, you can use two potions every battle, it doesn't remove two potions from your inventory. This means you don't have to worry about inventory management at all, in terms of running out and going to your local shop to restock. Of course, if you're just going to make a basic attack deck, then these cards are just a waste of space and time. Maybe throw in some weak potions in order to avoid long reloads, but the more valuable expensive ones would be useless in that particular case.
Friend Cards appear randomly during battle. Donald and Goofy will show up in almost every world and help by casting magic or attacking. Additionally, just like in the first Kingdom Hearts, characters from the world that you are in appear to help you as well. Aladdin appears in Agrabah, Jack Skellington appears in Halloween Town, etc. These friend cards are pretty useful, but since you can't really rely on them, they're more like special help than anything else. Use them when you get them. You should also stack multiples of the same attack together for better versions of the attack, just like magic cards.
Gimmick Cards appear only during boss battles. These cards always have a "0" value and stun the boss and expose his weak point. You usually only get one of these per battle, so either use it when you're ready to just completely unload on the boss, or when you need a break because you're about to die.
Enemy and Boss Cards show up after you beat a boss. These cards apply a status effect during battle to help you out. You'll get enemy cards randomly during battle instead of a map card occasionally as you defeat enemies in that world. You get boss cards after you beat a boss, pretty straightforwards. These cards can do something like give you wider attacks, increase your movement speed, or give you gradual healing. Other's are more radical and change all of your card values to "0". These cards can only be played one at a time, so you can't stack their abilities. These cards also have a certain duration, whether that's as long as "3 reloads" or "30 attacks" eventually they will run out. These cards cannot be reloaded with item cards, so use them wisely. These cards aren't a part of your main deck, so to play them you have to switch decks by hitting the select button and then playing the one you want.
Constructing a Deck and Leveling
Now you know how combat works, now for constructing your deck itself. Defeat enemies, get experience points, level up. Pretty straightforwards. When you level up you can either, learn a new sleight, increase your health, or increase your card points. ALWAYS CHOOSE THE NEW SLEIGHT!!! I don't know how clear I made this in the section about sleights, but sleights are super valuable. Maybe for the first couple of levels, you may want to focus on health and CP instead, but once you get to about lvl 7, you should always make sure you get the sleights. From there, you should try to get a good balance of HP and CP.
Card Points — Each card costs a specific amount of card points. More valuable cards cost more card points. The weakest kingdom key card costs about 13 CP and the Roxas enemy card costs about 100 CP. Higher number cards also cost more, a value "1" kingdom key costs about 13 CP and a value "9" costs about 18 CP. This is all important because the diversity and power of your deck is all a balancing act of the various strengths of the deck. Number of cards, specialty cards, the values of cards and the power of the cards are all factors to how you construct your deck. The exception to this rule are premium cards.
Premium cards cost as much as the lowest value card for that set, but are consumed upon use just like a sleight would. These cards can be reloaded using hi-potions, just like cards lost using sleights. For example, the lowest possible value of a "Three Wishes" card is 15 CP, the premium version of "Three Wishes" cards all have a value of 15 CP regardless of whether they have a "2" value or a "9" value. This can majorly cut down on costs for your deck if you use more premium cards, but at the same time, if you don't have enough potions, then you could potentially lose half your deck after one reload. You get premium cards by either modifying them in a premium map card room, or by receiving them in card packs from stores and by attacking the environment though they are much more rare.
Assembling your Deck
Finally, the last thing you need is the acquisition of the cards themselves. There are three ways to get cards, you can attack the environment (hitting lamposts, boxes, fountains, etc.), you can find them in chest rooms (key to rewards, calm bounty, or you can buy them using MP (Moogle Points) at a store. To make a store appear by using a map card, so you usually use one per disney world. You get a free pack of five cards, the first time that you meet him, and then you have to buy random packs from there. You can buy attack, magic, or item packs, combo packs that include a random assortment of all of them. Once you get to later worlds, different packs will cost more money, but will give you better cards. Enter a shop for the first time and you get a free pack of 5 cards, any other pack that you buy also has 5 cards. Unfortunately, you can't actually choose your cards, you just get random ones based on which card pack you buy. Thankfully, you can sell cards that you don't want, so I can usually get about half of the card points back that I spent if there aren't cards that I want.
Find cards, build a deck, kill bad guys. Buy, sell, buy again. Hack, slash, and unleash death. Combat in Chain of Memories is super confusing. You have to juggle card values, card points, enemy status effects, and enemy attacks. But if you can actually get all of this straight in your head, it works really well. It all sounds chaotic, but it's channeled chaos and that's the best kind. Hopefully you can use this as a guide to your experience. This isn't entirely comprehensive. It doesn't cover all of the nuances of Riku's gameplay, and it misses out on some of the other mechanics, but there's so much here already, it would be kind of pointless. I had a lot of trouble figuring out how the game worked, so this way you don't have to.