This guide mainly focuses on the basics and some other advanced stuff I have learned throughout the game.
I wanted to make a 100% guide but there are already a ton of other really helpful guides out there.

If you want to get 100%, here are some guides and videos that helped me with that:

https://psnprofiles.com/guide/6572-kingdom-hearts-rechain-of-memories-trophy-guide
Basically a solid 100% guide. The only missable element in the whole game is the gimmic card (The one with the green micky symbol with the value 0). They only appear in certain boss fights and the only thing you have to do is use at least one of them so that the gimmic card is registered in your journal.
Also: Dont flee from a fight and dont press "Continue" when beaten in battle. Just return to the title menu and reload your last save.

Note: After finishing Sora's story and watching the credits, Riku's story will be unlocked. In order to get every single card in Sora's collection, you need to finish Riku's story.
ALSO: You also have to have watched 358/2 to get even more bonus cards.
Which means: 358/2 days was originally a game for the DS. In this verion however, its just a sequence of cutscenes to explain the story. So you have to watch the whole "game" AND have to read every entry in games journal. In the menu under "Extras". Make sure to have read every entry. This includes Reports, secret reports, character entries and so on.

You will find 2 Chests instead of only 1 in the Rooms of Rewards, once you have seen everything there is to see in 358/2. The good thing is that it doesnt matter at what time you finish 358 days. I have finished it before playing CoM. But for the stories sake you might want to finish CoM, then finish 358 and finally collect the remaining cards. Keep in mind that you will have to generate the rooms again if you do the latter.

My order was:

• Finish Sora's Story (make sure to get the gimmic card)
  
• Finish Riku's Story (Finish every riku related achievement [also gimmic card] and save on an empty slot: DONT OVERWRITE SORAS SAVE)
  
• Finish 358/2 Days (This can be done before playing CoM, if you want)
  
• Get the remaining Cards and clean up other remaining achievements

There are some grindy - stupid achievements like the the "Room Generator" or the "Expert Deck Builder" which are not naturally done in a playthrough. At least, it wasnt the case for me.

https://psnprofiles.com/guide/287-kingdom-hearts-rechain-of-memories-boss-guide

https://www.youtube.com/watch?v=lsHpXR8CwGU
https://www.youtube.com/watch?v=BhYVHunPal4&t=0s
Worked like a charm for me. Basically just watch the entire video and follow every step exactly like in the video explained. This includes the World in which to farm, the deck build, the card order, the number of roles and which enemy to either attack or not attack. Most Enemy Cards popped after the second try. Super helpful and saves a ton of time.

World Cards:


With these cards you can enter new worlds. You can also return to already visited worlds with the black glasball thing in the floor lobbies.

You can basically choose your own Order but I recommend this:
Sora:
Traverse Town > Agrabah > Olymp > Monstro > Halloween Town > Wonderland > Atlantica > Neverland > Hollow Bastion > Destiny Island > Castle Oblivion
Riku:
Hollow Bastion > Agrabah > Monstro > Traverse Town > Neverland > Olymp > Wonderland > Atlantica > Halloween Town > Destiny Island > Castle Oblivion

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Room / Map Cards:


Room cards lets you navigate through the world. Every time you want to generate a new room you must meet the required value and/or color: For example 5+ Green: Only green cards with 5, > 5, or 0.
You can re-generate open rooms as often as you have cards and the requried value by pressing "Y / Trianlge" on an open door. This is especially valueable if you want to either grind / farm or want to use a number of bounty / treasure / Mogry cards back and forth.

Room Cards come with different colors: Red, Blue, Green, Grey, Gold.
Red, blue and green are the cards that let you generate a new room with special bonuses. The Grey card is a "Joker" Card which acts like a joker for every requirment for a room (For example 99 red).
Gold cards are either Story cards, which you get automatically or the Rewards cards. Those only drop on the seventh floor and onwards (in my order Atlantica and so on).
Reward cards do not stack. You can only keep one until you have used it. But in the end game worlds (Destiny Island +) the droprate is quite high, so dont worry too much about it.

• Make sure to hit or jump on all kinds of objects in the world. They often drop health, mogry points or even cards.
  
• Make good use of the games map to navigate through the world.
  
• Use at least 2 bounty cards in every world. The first one usually gets you a new sleight and the second a new attack or magic card.
  
• Also: Use at least 1 mogry card in every world to strengthen your card collection even further. You get one stack of cards (5 cards) for free and can buy up to 16 new stacks per generated room.
  
• As stated before, you can re-generated rooms by simply interacting with an open door. You can either farm level or get as many treasures as you have cards available.

Attack Cards

With the attack cards you execute a simple attack. There are different kinds of Keyblades that do have different attack properties like Swing Speed, Break Recovery, or elemental damage. If you dont want to get to deep into this, dont worry too much about it, but here is a link that further explains the keyblade properties:
https://www.khwiki.com/Attack_Card

Magic Cards

Similar to the attack cards, magic cards (Blue Border) execute a spell like Fire, Thunder or Cure.

Summon Cards

They are the same as magic cards (Blue Border). Just want to showcase them as they are NOT friend cards.

Item Cards

Item Cards (Green Border) are there to reload a type of card. The Healthpotion for example reloads your Attack cards (not the first card in a used sleight) while the Ether reloads your magic cards.

Friend Cards

Friend Cards can only be obtained during battle randomly. They call party members in the specific world and can be used in a combo / sleight.

Enemy Cards

Enemy Cards can be equipped in the deck, but are played in a seperate deck menu (PRESS SELECT) during battle. They offer temporary passive effects. Jafar is the most useful one as it prevents enemies from breaking your attacks for 20 attacks in total.

Premium Cards
Premium Cards are the cards with the yellow value or the shiny cards in the card collection preview. The point of premium cards are the reduced CP cost, which allows you to include more cards in your deck. They dont do more damage or anything else.

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The combat is really something to get used to, especially if you have just finshed the other KH games where you basically just smash the attack button. In this game its not that easy, as you get cancelled every so often by the enemies cards. This is because of the value of the played card.
Card Values range from 0 - 9. The higher value always wins, on equal value, both attacks get cancelled.
The 0 Value card is a special one as it breaks every card value and attack but gets cancelled by every other value if used as an attack itself. Meaning: Use 0 Card values to cancel enemy attacks, be it heartless or boss attacks.

Sleights / Combos
It is an absolute must that you learn how sleights and combos work in this game. This goes hand in hand with the deck building which I will explain later on.
Basically, a sleight consists of 3 cards. It can be 3 attack cards; 1 summon card and 2 attack cards; 1 magic, 1 friend card, and 1 item or whatever. The values add up and are therefore harder to cancel by the enemies.
In the deck building menu you can see the sleights by pressing "X". Those are the sleights that you have learned by leveling up and choosing the 3rd option (Recommend to always max out sleights first), getting them throughout the story, or obtaining special sleights in bounty / treasure chests.
For example: Stun Impact can be obtained by level 7. The requirements are 3 attack cards of the same type (keyblade) and the accumalted value has to be 20 - 23.
However magic sleights dont care about the value. Firaga uses 3 fire cards of any value.

Keep in mind that the first used card in a slight will disappear from your deck during battle. Because of this, it is usually better to start a sleight with a low value card, so that you dont loose a high value card on your next reload. They can, however, be re-obtained with high quality item cards.

The most value tipp I can give you here is that magic sleights are absolutely broken in this game.
Throughout the game Blizzaga (3 Ice cards) has been more than helpful. Also early obtained Summing and friend sleights were very helpful.
Another thing worth mentioning is that using the shoulder buttons at the same time will let you use a sleight without having all 3 cards. This is only helpful in certain scenarios, but still worth knowing.

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The deck building is another element that is way to important to ignore. The reason is that the order you place the card WILL ALWAYS BE THE SAME. Maybe Im the only one who thought that the order doesnt matter and the deck gets shuffled in every battle, but thats not the case. This is super helpful, as it allows you to prepare powerful sleights and execute them right at the start of the battle (like Blizzaga).
My very first deck build was "Stun Impact" at the beginning to stun the enemies, distance myself a bit and then throw in a blizzaga to do heavy damaga on a wide range. If there are some enemies left I finish them with attack sleights like sonic blade.
In the late game, where I focus more on leveling to get to 99, I have a mix of Mega-Flare and Tornado which kills everything in the first 2 sleights.

When deckbuilding, keep an eye on your deck points. Every card has a card cost and you cant go past your deck points. Deck Points can be increased by leveling.

I also highly recommend to build 2 different decks simultaneously (you can have up to 3 decks at the same time). The first deck for the heartless which is mostly AoE damage based and the second deck is used for boss enemies as the focus of this deck is to stagger them and do a lot of damage on a single target. Dont forget to switch and save decks before entering story rooms. Also, once you have jafar, I recommend adding him to your deck, as he prevents the enemy to break your sleights once activated.
More infos on boss deck in the boss guide I have posted in the "Beforehand" section.

If done right, you wont have any trouble with most bosses.

Also, make sure to have cure magic, item cards to restock your cards AND Cards with value 0. I highly recommend placing them at the end of your deck and also assign a shortcut to access the cards with a "D-Pad left".

Once you have finished Sora's story, you will be granted access to Riku's story and his perspective during the events of Castle Oblivion.
Riku's combat reminds me more of the usual KH as you will smash the attack button once again.

Special Riku rules:

• Riku cant edit his Deck because the deck is pre-generated for him in every world
  
• Riku does not need to hold the "Reload Card" to regain his cards.
  
• Riku cant heal. He can only heal by using the micky friend card.
  
• Riku can "Duel" enemies, which you will learn in a tutorial in game. The tut tells you to choose the card with L1 and R1 but the timer is just to short. Just spam attack.
  
• Riku can enter into dark mode, which is basically the boss battles in a nutshell. Get into dark mode and use sleights to finish the enemy.

Riku's story is way shorter and IMO way easier.

Some additional tipps regarding Riku:

• Make good use of the enemy cards, especially Maleficent, Parasite Cage and Jafar. Keep in mind that only 1 enemy card can be activated at a time.
  
• Make sure to always level up AP first.
  
• Enemies dont drop cards. Tbh i dont know how I got most enemy cards in Rikus part of the game but at some point I just got them all.