Successfully complete Chapter 7 to unlock the "Gallery" option and Dororo mode.
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.
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.
At Chapter 7, after you defeat the demon Tahoumaru, go back to the place in Daigo's Castle where you defeated Tahoumaru. You will find a very special sword: "Sword Of The Eight Cloud". This sword allows you to make crazy slice combos. With this sword, make a slash attack (hold Triangle). Then, for every slice you make, the CPU will count 3 slices. Note: The button combinations are harder as with other swords. You can easy make 60 or more slice combos. With this sword, you can easilty get very rare swords or objects in the game, and defeat the final Boss slightly easier.
First, The boss will fly around the outside of the area, shooting fire balls at you. As you move to get out of the way of them he will stop just long enough to get one shot from your rocket launcher. As his health gets to the red, he will shoot out more fireballs, Wait for the right time to shoot him. Next, the Boss will come up to the side. This is one of the easy parts of the fight. He will start to move a hand back and forth across the ground. Stay to the side to avoid it. Hit it once or twice to get it to stop and jump on it. You will then be brought to his face. That is where you can do damage to him. He will shoot a lightning bolt at you if you do not hit him enough times. He will then throw you back. Repeat this until his head splits in half. Next, the Boss will try to stun you with lightning bolts and bursts of air when he takes off and lands. Try to keep from being hit as much as possible -- if his head if facing you when he lands, he will shoot out a beam. Try to stay to his side or back when he shoots out the beam. You must then run up his tail and onto his back and attack the spikes back there. Try to destroy them as fast as possible; the hand at the end of the tail will try to grab you. You can move out of the way, but if it gets you it will try to throw you into the beam. You can jump out of the throw. However, be sure you go in the correct direction so that you will not hit the beam as you destroy the spikes. There are two layers for each one, and for each one you destroy you will remove one of the lightning that you will have to watch for. In the next part of the battle, he will start dive bombing you. Only use the sword and nothing else. You must do a combo to stop him, and this works best with just the sword. With each hit you will slow him down, but get pushed back yourself. After you get the hits in to stop him, what appears to be his heart pops out. Charge up the sword and do a 48 slash combo on him.