How to get more Spirit to upgrade heroes?

There are a number of ways to get more spirit to upgrade your heroes. The best place is in the campaign, as you complete each level you'll get some spirit, plus the auto battle sequence will give you quite a lot if you've been away from the game for a number of hours.

You can also get a ton fro the Marketplace, but you'll need to use gems or coins. Other places to get Spirit include the wishing fountain, daily quest and event raid.