The requirements for this achievement are self explanatory, but the rules of the game can be a little obscure if you've never played before. I'll go through it all in detail. The overview section is explained very clearly in the game, but I'm including it for the sake of completeness. You may want to skip to the Mechanics of the game section.
Overview of the game
You win the game if at least 6 of the cards on the table are blue. You lose if 6 or more are red. If both you and your opponent have 5 each, the match ends in a draw and nothing happens.
You "win" cards on the table by claiming them outright or by battling them. If one of your card's arrows is pointing toward another card's arrow, a card battle ensues (more on that later). If your arrow is pointing towards an empty part of your opponent's card, the card turns blue with no fuss.
If you win, you can pick an opponent's card to keep, so long as you "won" it whilst playing. Similarly, if you lose, your opponent takes one of your cards. If the score is 10 - 0, the winner takes all of their opponent's cards.
If you lose one card, you can probably replay the NPC to reclaim it and register a win in the process. If you suffer a 10 - 0 defeat and lose your whole hand, I would strongly consider reloading your game, as building a decent deck can take time, especially if you've been levelling up cards (more on this later).
Mechanics of the game
Card battles can be the most confusing aspect of the game, often differentiating between victory and defeat. The mechanics of card battles are not made explicitly clear in the game (though you can find some strong hints if you go looking). Let's break it down.
Each card has 4 values: For example, your card may read "3 P 0 1". What does this mean?
3: This number is your card's attacking power. The values range from 0 to A, with the value switching to the letter F instead of the number 10.
P: This letter denotes your card's attack type. There are four attack types:
- P: physical
- M: magic
- X: cross (pits your card's attack power against the lowest defence value on the card it is attacking.
- A: assault (pits your card's best stat (attack or defence) against your opponent's lowest stat.
0: The third value denotes a card's physical defence.
1: The fourth value denotes a card's magical defence.
When you initiate a card battle, your attack value is pitted against the other card's defence value. If your attack type is P (physical), then your attack value (the first value) will be pitted against the other card's physical defence (its third value). Similarly, if your attack is M (magical), your attack value will be pitted against the other card's fourth value.
The X and A attack types can seem confusing at first, especially A. In reality, it is quite simple. A attack type cards simply replace the first value (attacking power) with the highest value on the card.
For example, your card has the following stats: 0A83. Because your card is the A attack type, your attacking value is not 0, despite what it says in the card. Your attacking value is 8, because that is the highest stat on your card. If your card is the A attack type and the attacking value is the highest value (8A20, for example), then the attack value will remain at 8 and will be pitted against the opposing card's lowest stat.
There is an element of RNG to this game, unfortunately. Just because your attack value is higher than your opponent's defence value, it doesn't mean you will win. When cards begin battling, they are assigned a random number based on their value, and there is significant overlap of the random number ranges from each attack value.
Watch out for combos: If you win a card battle, the card you have just won will turn all other cards its arrows are pointing to in your favour. This also works when your opponent wins a card battle. A common tactic used by NPCs is to play a very weak (0P00) card in the centre of the board that has many arrows, and they will win your superior cards by winning a card battle with the weak card after you've turned it. Count the arrows, and plan accordingly.
Levelling your cards up: every time you win a card battle, there is a small chance the victorious card will level up. This means any of its stats can increase. Most importantly, it can go from being a P or an M attack type to an X attack type. X attack types can level up to an A attack type, which is ideal.
Based on this, cards like Genji Glove and Alexandria (both of which can be obtained by playing the jump rope game at the beginning of the game) can become exceptionally powerful if they win enough card battles and upgrade to A attack type. As previously discussed, even though they appear to have 0 attacking power, the A attack type means their highest stat (probably physical defence) becomes their attacking power, which makes them very strong indeed.
That's more or less it. If you have any questions or comments, please post them here so other people can benefit. Also feel free to PM me if you desire.
As long as you play every NPC you can play at least once as you encounter them, you'll quickly find a deck that works for you, get good cards and be able to level them up. When building your deck, make sure you strike a balance between arrow coverage, offensive and defensive cards and make sure you include both P and M cards.
I'm not sure if you can rack up multiple wins against the same NPC and have them county. I think think the achievement requires beating 100 different NPCs, but as the description states you can rechallenge some of them after certain points in the story and it'll count as another win.