As others have said, your units max out relatively quickly and after they're maxxed out all the xp they obtain is rendered useless. This problem is highlighted with your hero unit. Im just about halfway through the trials and my hero unit has ~ 4k of unusable xp. By any chance could we have this excess xp go towards upgrading other units or at least serve a function?
 "Epic War 5"){kind=icon}
 "Epic War VI"){kind=icon}