Could it be you're doing double damage because of your Driving Blow special skill that your main uses?

Can you post the statistics screen of your main hero? (with no gear or runes, and with complete gear and runes)

Thank you

Not everyone will go 10 stars. 750 gems for 10 stars is extremely lucky. I know lots of people who have spent thousands of gems and only got 6 or 7 stars maximum on one hero.