PRO Codes were added in KH3 via the ReMind dlc update on console but we get to have it at launch thanks to being on PC. I'm going to skip the formailities of explaining what PRO Codes are. The main thing is that PRO Codes act as a challenge that can be opted into to make the game harder. Commonly by disabling specific features in battle.

Every PRO Code has a unique amount of stars attributed to it based on the general difficulty seen by the developers. This is not a random choice but the amount of stars is important to the Merit points you will receive upon defeating a specific boss/encounter. Notably, every star adds a 1.25 times multiplier to the base pro code amount. Therefore if you were to beat a boss worth 100 points with one star, you would receive 125 points. (You cannot earn 100 points from a boss as you always need one star to earn points.)
If you desire a formula, you can use X*Y*1.25. The X value should be seen as the base point value for the boss and Y should be the amount of stars you have active. The Y value should never exceed 40.

Now that stars have been explained, the next step is understanding Points. The amount of points you will want at the end of your playthrough is 364,125 for the A Rank Merit Badge.
Not all forced encounters will give you Merit points for the win so you can turn the codes off after you fight the boss and then turn them back on before the next one.

The amount of points every boss can provide is listed in the table below.

This next section will be associated tips for playing with various PRO Codes.

No other way to say it but to just get good and use high defense/resistance armor and high strength/magic accessories. The best way to work with Zero defense is resistance armor and not defense armors. Resistance is much better than flat defense if you know the element you're going up against. In the keyblade graveyard/LimitCut, the best bet is usually high dark resistance armor with a few exceptions for bosses that don't use darkness as much. Vanitas you should use ribbons for due to his various elemental attacks. You should use every ribbon you can in the case of fighting Master Xehanort. For fighting Xion, you should use high defense as the only magic she uses is light based which has no resistance armor. You should be able to figure out what elements various bosses use.
HP slip will never drop you below critical health (the beeping sound will be playing). If you have Second chance and Once more Withstand Combo active as abilities, you should be fine. MP slip is trickier especially when combined with other codes as a longer MP recharge time makes healing difficult. Block Replenisher will be useful for maintaining MP. It will not decrease MP recharge time though.
Airstep still works. Use it to exploit openings easily.
No cure is easier to deal with in my experience because elixirs are so good. If you are using both, healing will need to be done through other means. The only particularly consistent method is acquiring 2-3 wisdom rings and equipping them. If you only have two, you should equip Grand Chef as your secondary Keyblade. Wizard's ruse, when you have three equips of it, will give you a 50% chance to restore health based on how much mp you spent to cast magic. You can use either big spells or small ones. Big spells will obviously restore more and small ones give you more rolls for healing. It's your preference. I prefer big spells but it's useful to have your second and third shortcut pages to have the tier 2 and tier 1 version of your spells so you can heal in a panic. For MP you can restore it using Block Replenisher on bosses with multi-hit moves. It will not decrease MP recharge time though.
Just don't use them. Don't rely on them. And Remember to unequip their associated abilities if using the Ability Limit Code
This code completely disables all situation commands except for certain special ones like showdown used in the data Xigbar fight. If using this code, the only keyblade you should really use is ultima weapon as well as Grand chef if using the No cure and No battle item codes. All abilities relating to them should remain unequipped if using the ability limit code. Notably do not equip Critical Recharge and Critical Converter as those will have no effect with this code on and it will use up your ability slots. Extender abilities should also remain unequipped.
It will suck for some bosses considering how high some health pools can be in the base game because it expects you to use attractions. I just recommend turning it on only in Remind, LimitCut, and the Secret episode. Of note is that the 10k heartless fight in keyblade graveyard will still give you magic mountain train so don't worry about it there.
Less massive aoe nukes for free.
Your Kupo coins won't work so don't buy any unless you plan to donate them to someone who will take it.
READ EVERY ABILITY CAREFULLY.
If an ability interacts with a PRO Code you have on you should leave it unequipped. If you can't use cure, don't equip leaf bracer. Don't use flowmotion abilities either as those suck in combat. Don't equip aerial dodge and use air slide instead. Mobility abilities are usually good besides aerial dodge and flowmotion in boss encounters. Combo modifiers to avoid are Falling Slash, Triple Rush, and Magic Flash. You should always have on are Second chance and Once more Withstand Combo. Especially in the DLC.

Information I got about PRO Code points and how they work was from KHWiki[www.khwiki.com]. KHWiki is a comprehensive wiki that does not use fandom's site so it is a comfortable browsing experience. You can typically find all kinds of information on there whether it be more detailed breakdowns of abilities and equipment, Boss strategies, and other things I did not mention in the guide.

Thank you for reading the guide and please Rate positively.