Quick sword level up
Find a location that only has a portal that lets out birds with nothing else around to interrupt you. As soon as the birds appear, kill them with you arm swords, but make sure that you do not hit the portal or it will be destroyed after a few hits. The left arm levels up more quickly than the right arm. However, with some time you can get them leveled to a decent level at the start of the game to help you in the Boss fights that happen later.

Raising Dororo's HP
As you play the game, notice that Dororo gets a heart after killing a demon. The heart increases his maximum HP. In Chapter 4, when you play as him for the first time, you will encounter three portals that put out wolves. Destroy two of the portals and focus on the wolves that are exiting from the last one. Every two or three that are killed will drop a heart. Make sure not to destroy the portal until you have all the health that you desire.