While the Heroes page has several suggestions to make your heroes more powerful, here are a few specific tips for gaining levels for them.
- Spend Diamonds on Stamina so that you can do more Exploring
- Use your Training Slots wisely!
- EXP Potions are your best bet for quickly leveling (see link for details)
- Your fellow guild members can use the Training Camp to boost the EXP of up to 2 of your Heroes or Angels each day