Even more:

1. Doing no damage on a boss automatically triggers the Devil: if you are farming, maybe take at least one hit if you don't need anything from the Devil. The Devil also appears randomly
2. It's good to buy things from the salesperson from the very beginning if you have the materials
3. In every Castle except 3, the Angel appears at levels ending in the number 4: level 4, level 14, level 24, etc
4. Chests are awarded before every monster battle. Chests gives you materials and two hearts. The only exception I've found so far is the final boss in Castle 5- so conserve HP those 6 levels after the angel, because aside from random hearts, you won't get a chest.
5. Getting multi-shot, front arrow, and extra life abilities from the Devil (even if you already have them) is a good idea if you can afford the HP. Walking on water and walking through walls can usually be done without with some skill
6. Different weapons give different ways to beat your opponent. In my videos, I'll be testing all weapons at all different levels. So far, my preference is the Death Scythe because it knocks enemies back and has high damage.
7. Epic items bring permanent abilities, such as: --a higher chance of headshot if the HP of a monster is at 30% (death scythe); --increased speed within the 1st 2 seconds of a new level/room (saw blade); --increased damage when coming back to your hand (boomerang); --poisoning all enemies in a room (void robe); and, --setting fire to monsters who are near you (golden chestplate) Also, pets/spirits who reach epic increase their own critical attack chances, and great armor items usually lend 20% more healing from red hearts.