Is paying real money the only way to level up in this game?

When I'm playing, suddenly the opponents get super strong, and it feels like they summon more often too. Even if they're not invincible, I'm starting to think I might need to spend money ㅠㅠ I'm diligently collecting coins and items by watching ads, but since it's just a game to kill time, I feel like spending money is a waste ㅠ