Maneuver a Boss against a wall, then keep hitting it with your sword.
When you are low on money or hearts, if you have the shovel, just start digging in alot of areas. This is an easy way to get $999, and maximum amount of hearts. This will not work in all areas, but it will work in most.
To kill bosses easily, trap them against a wall and attack them with your sword!
You must have atleast 30 jewels for this to work. When you've only got one heart left and you're near a save point, save the game, restart, and load it. You'll be where you were with a full complement of hearts.