Watch TV Royale
Clash Royale is a continuously changing game. Every month the developers make certain cards much different. By adding or removing strength or damage, or changing the dynamics of the troop. The goal with this is to keep the game balanced, while breathing new life into battles by making older cards more viable all the time.
However, this means that after each balance-changing update (monthly) millions of players need to change their decks if they still want to win. Even if your cards didn't get changed, others could now be more effective.
Sadly most don't have time to read through pages and pages of forums, or battle in challenges (or vs the trainer) for hours on end to see what works, and what doesn't. Our biggest tip would be to watch TV Royale. This shows replays of other players based on arena. You'll see some of the top players in the world, or just average people that made it into the replay system. However, it helps you see what works, what failed, or even how someone with your exact deck won their battle. Try to find one similar to what you use, and make the appropriate changes to be more successful.
I'm constantly scanning TV Royale for a better deck. Remember though, some of those guys have used the same deck for months on end, and have it mastered. Meaning you may need to practice with it first, before you'll be as good with the same selection of cards. I always test a new deck vs the Trainer, in a friendly battle with clan members, then in a challenge before heading into a regular multiplayer matchup. We'd recommend you do the same.