If you don't understand which characters are good against the different monsters read this and it should give you a better idea of how it works.
When you fight monsters they have 1 of the 5 colors: purple,yellow,blue,green, and red. If a monster is blue you should pick all the blue puzzle pieces because it will do more damage than all the other colors. If there is a red monster and a green monster you need green and red puzzle pieces if you want lots of damage. I hope this helped you with the game and if it helped please like this comment.
Nevermind, got it to work. Keep up the good work on this game. If there is anything to improve on you should try it with seeing info on the cards. Like when I click on my card to see its strengths, try to make it not move an inch at a time. Try to make it pop up in one sequence.
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 "Android Soccer"){kind=icon}
 "Idle Recruit"){kind=icon}
 "Dangerous adventure"){kind=icon}
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 "Blade Hunter"){kind=icon}
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Thanks :)