Assuming you have already completed Chapter 4. I have found the best way to stack Hero Rank points is to choose an Element your team is strong against in Chap. 4, and just farm that elements Dungeon as they give a chance to drop Ingredient items (harps, youth fruit). Also, if you need a specific harp, farm that element types dungeon, it will eventually drop.
Not a bad idea. Thanks for this!
Another way (cost you some gems) open with silver key pyramid and play it few times every game you get 4K rank points.
Yes, that’s the way how I upgrade my hero.