I just finished it for the second time in Heroic mode... Heroic mode in places was barely harder than Easy, if not easier (the first chapter or two were substantially easier than my first time through with no weapons and no level development). But it got fairly tough in the later chapters, and the last few segments of the Underworld were quite difficult. There were still maybe three crystals total I did not find.
Oddly, some of the "hints" during loading were kind of silly. I got told during heroic mode that I could go back and play the game in Heroic mode, which would be unlocked once I finished the game.
Only other qualm that came up during the replay is how difficult cycling through weapons is. The weapons select button is not that sensitive, since it's small and in a corner, and It's a major nuisance to have to retreat and try to find safe space while you're cycling through weapons. It would be nice if you could select your weapon while the game was paused -- I don't think you can. There are lots of situations in Heroic mode where I find it beneficial to alternate between the Axe and the Bow (I love the dual blades, but they just don't cut it against the enemies that have physically longer weapons like the minotaurs with clubs and axes). Unfortunately, cycling from Axe to Bow entails going through
every other weapon as an intermediary.
Anyway, supremely satisfied. Excellent game at $9.99.