If you are low on health, try to find some weeds and chop them down.
Notice that your Rage Ability switch will gradually increase. Keep chopping down weeds and you will soon be able to use your "Ninja Revive" to heal yourself.
Get stronger swords:
To get stronger swords, you have to kill enemies. Notice that when you complete a mission (at the blue screen with Ninja floating), there is a sword at the bottom of the screen called "Ranx Bank". The sword has numbers that increase with the more "Ranx" that you kill.
Also, the more "Ranx" you kill, the sword gets covered in Ranx blood. When the sword is full of Ranx blood, at the "Mission Complete" screen, Ninja will be holding his old sword. He will drop the old sword and a new one will fall into his hands.
To unlock the Battle Arena, get all the grades. You should then get the Master grade. The Battle Arena will now be open in Robot Beach.
In the first part (where there is lava for a field), keep a steady pace while jumping around on the stones. Hold [Forward] and the # direction that you want to jump and collect the shuriken. Throw it at him without locking on. After throwing the full amount of eight shuriken three or four times, he will fall down into the lava.
Then, he will jump up and absorb the lava. You will now have to sword fight him. The Soul Sword and Revive Rage are recommended for this. If you run at him and slash repeatedly, you are going to get hammered. The best times to attack him are when he just finishes a quick slash and when he is just about to do the non-shock wave ground pound. Hit him at those times and use Berserks and Insane Rages every time you do not need to revive.
Attack his belt. After you attack it twice, he will slam his fist. You can now attack it two more times before he slams you away.
Notice that the sword color changes in the your belt changes.
1. Ninja's Sword
2. Golden Sword
3. Jade Sword
4. Sapphire Sword
5. Fire Sword
6. Soul Sword
First skill - Ninja Berserk
Second skill - Ninja Revive
Third skill - Ninja Shuriken
Fourth skill - Lightning Rage
Where there are grass, flowers, mushrooms, other foliage, objects other than crates and barrels, and certain decorative objects, you can cut them down to earn a few extra points towards your main score.
This is useful in the certain levels that require a high score to earn the grade.