This is a popular question among Btd players. There are many ways to unlock everything in the game.
1. If you have all the characters unlocked, then this will be easier for you.
2. Use coins and diamonds to buy items from the shop like new weapons, armor, etc that can help you gain more experience points in battle.
3. Defeat bosses and collect keys from them to open locked chests with more valuable rewards such as potions or rare items like coins or gems inside them.