Hi! I beat CoM years ago. If you wanna play effectively, don't throw in strong cards, but look at their values. Strong cards with low values are useless.
When fighting, you can either choose combo or sleights. For combo, just put 0's in the first slots and 8/9's in the second and finisher slots. When the enemy uses a card, you just start from the 0 to break it, and use the other two to finish your combo. But the cards could get broken easily if the enemy uses sleights or 0 cards, especially in later stages. On the other hand, sleights work more effective, as they grant special effects and do more damage. If you don't have enough CP, use cheap sleights like Ars Arcanum. If you haven't learned that, though, you have to depend on your current sleights. There are also a lot of hidden sleights that you need to unlock by obtaining certain cards. Check
here for more details.
My advice is you gain enough HP (around 200) first by leveling up, then only you focus on increasing CP and learning sleights. When you have enough CP, put in suitable enemy cards, like Jafar. Heartless will drop their corresponding enemy cards if you finish the battle fast. You can go check their effects
here. They really help a lot, and can even make you almost invisible! Also, make different decks for bosses and ordinary enemies. Before fighting bosses, create a deck that aims for their weaknesses (most bosses have weakness). To increase the chances of getting high valued cards, break every barrels / jump onto every platforms / hit every bushes that gives you prizes. You get more cards and MP this way. If possible, put in some item cards, because the reloading time could kill you. They cost a lot, so you gotta level up and gain as much CP as possible.
When synthesizing rooms, first think of your objective. Use Premium Room to make as many premium cards as possible, and never use Almighty Darkness unless you are looking for enemy cards. Open at least one Calm Bounty in each world. That's all my tips by summary. If I go into details, I will have to type a very long post. Best is to check out the walkthroughs and FAQs.
CoM was my first game, so I didn't feel anything wrong, but I really had a hard time when I first played it. I think this system is kinda unique and I like it. If you're playing ReCoM, you're lucky. The screen and controls are easier, and it introduced more sleights, sleights that really make bosses easier than Shadows. If you put in correct cards, the enemies can't even touch you. But this is just for Sora's story. Once you complete it, you have to play Reverse/Rebirth mode, which is harder. That, however, is another topic.