1)  Never buy a card if you don't want to spend more money on the game, Renting cards is the cheapest way and profitable to earn more in this game.

2) Never play so many rank matches, The reason being it will reduce your DEC capture rate so you only earn less DEC per win so always keep your DEC capture rate above 70% so you don't have to play so many matches to earn a decent amount of DEC per day.

3)The DEC percentage will increase up to 20% per day if you play 4 hours per day, so you don't have to worry if you have a low DEC capture rate, It will replenish overtime but try to maintain it above 60% at least.

4)Don't rent card's if you are below 1000 rating the reason being you can easily push above 1000 rating using the starter cards above 1000 only you face some difficulties and you get a better understanding of the game if you follow this rule (Trust me it will make you a better player)

5)If you keep losing don't get frustrated instead analyze your opponent's deck because every level has a certain meta based on the player's skill so you need to adapt to that by selecting which deck the opponents are using.

PS: If you want to know where you can find the DEC capture rate follow these steps, 
1 Just click your DEC  balance 
2 Below the balance you can find the DEC capture rate