Things are coming together. I'm able to play through the entire game with only a few outstanding issues. I'm fairly satisfied with the difficulty curve of the game, now to just balance individual skills and weapons.
Here is a gameplay video of all four boss fights:
Gems on the board get swapped spontaneously because of special hats worn by the heroes. For example, the pirate's hat changes 1-2 gems to coins each time damage is received.
The fourth and final boss may seem weak, but that scythe attack does random damage so luck plays a role. Plus, only two heroes can be part of this battle, unless the war banner trinket is used for an additional hero. The tortoise is very useful here, because of the ability to temporarily polymorph any enemy into a harmless sheep.
A general strategy is to pick and choose heroes, hats, and weapons that turn gems into a specific type to feed a particular hero for special attacks. To avoid monotony, some stages feature enemies that are immune to certain damage. However, the jagged blade that removes all armor of an enemy will eventually allow any strategy to prevail.