This guide is written by a 292.4 hours player who have the winrate around 60%, which I think it is still very low.
No save scamming used for the winrate.

This game is based on Blackjack, which two players draw cards that have points close as 21 as possible, but not exceed 21. In this game, the player have more points deal damage to the other player by the points difference, and if one of their HP goes 0, the battle ends.

Busting will make it count as 0 while comparing points, which often cause them to take lots of damage unless the other player busted too. Hitting Blackjack can activate the powerful blackjack effect based on the suits of the cards.

Based on those two, there are three vital strategies in this game, ordered in their importance:

• Avoid Busting
  
• Aiming Blackjacks
  
• Force Busting

So here is the most important one out of those three, Avoid Busting.

Here is a very common scene that most beginner would played wrong:
You: 12 points
Enemy: 19 points and has stand
In this case, most new player would hit but that is the wrong choice. This game punish the player who busted a lot. In this case, if you are lucky and not bust, the maximum HP can be saved is just 19-12=7 points. If you choose to stand then you lose 7 HP, but if you bust you would lost 19 HP. Which choice seems better now?

For the starting deck with A2345678910JQK, you may always choose to Stand while you have 12+ points to avoid bust. Of course, you can still hit if you have an Ace that can change its points so you would not bust. In the other hand, sometime you may find your enemy got a blackjack will full of clubs, in this case, you would take lots of damage even if you did not bust, and the only way to defend it is to get a blackjack to tie off it. So sometimes it is better to take risk.
In the late game, you would have many ways to save you from losing too many HP if you bust, such as picking cards like Get Out of Jail card. Also, you may also pick lots of Negative cards, which help you from playing lots of other cards without busting.
Point-changing exploiting cards such as Dark Flower, Transit Card, Priceco Membership Card, Queen of Chess can also help you to avoid busting by exploiting them to decrease the points.

Aiming Blackjacks is WAY more important than many players think. In this game, if you hit blackjack, you can either heal, shield or deal more damage to your enemy, depends on the suits you got. There are many ways to aim the blackjack, but here are the most common ones:

• Flexible cards such as an Ace. Flexible cards can give you a second chance if you hits and fail to get blackjack with the drawn card, without busting you.
  
• Round-up cards such as a Pi, Half and other cards like Misprinted Card, Jack and the Beanstalk all works. This make you need fewer points to hit the blackjack while you can still hit it by the normal points needed.
  
• Handy cards such as Credit Card, Wild Card, Kanban Card, Ace Up Your Sleeve and many more. They can be played anytime, which is perfectly to be played when they can give you a blackjack, like you have 11 points and you have a Credit Card to let you go to 21.

In the late game it is the vital way to prevent losing too many HP using the Hearts and Spades. It can also used to deal damage using Clubs since the late-game enemies often stand at points like 19 and 20 without busting.
The suit called All at Once, can give you the effects of all 4 suits at once if you got blackjack with it, effictively giving a card that worth about 30 points while only taking 10 points on one card.
Jack of All Trade, King of Kings and Wild Card have this suit.

So I said the ways to defense, here is the way to defeat your enemy fast and safe by busting them.
There are also many ways to do it, such as:

• Giving Cards to your enemy by using a card that can move to your enemy's side, such as the Four Mana Seven Seven, Queen of Chess, Jumping Jack and AI Generated Card.
  
• Increasing their points by using point-changing cards like Break Limits and The Victim Card.
  
• Max-points changing cards like Rule Card and Sweep the Leg, which make they can play less cards per turn and have higher chance to bust. Also, if your max points is greater then your enemy, you will always get the blackjack without worrying to be tied off unless the enemy changed it again. That is vital to defeat the Act 5 enemies and the Final Boss once you unlocked the All-in difficulty, which the enemies always changes the max points at the start of battle, if you can not deal with it you would have very hard time to defeat them.

Here are some other techniques that help you for winning:

• Burn and Steal cards from your enemy by using cards such as Killer Queen and Library Card
  
• View your and your enemy's Draw pile often, so you can tell it is better to hit or stand. If there are only 1 card left you will always play it if you hit.
  
• The benefit of each suit : All at once >>> Hearts > Spades > Clubs >>> Diamonds > Nothings
  
• Rating for every deck, which helps you from understanding the decks a bit: https://steamproxy.net/app/2400510/discussions/0/601910394159660758