This boss isn't difficult at all. Follow this guide and you'll have no problems.
Ninjas 1/3 -
To start off he will spawn a ninja with a flaming sword, move quickly left and right while slashing at him, after he swings at you, he will pause for a slight second, slash him as quick as you can until he starts moving again. Repeat this until he is dead.
Boss 1/3 -
There will be a ball/orb connected to the boss, slash at these while avoiding the lighting he shoots at you until it breaks. Once it breaks, throw a ninja star at him and repeat the process. You will then have to dodge his attacks.
Ninjas 2/3 -
Now there will two ninjas just like the first stage, and this time your health will drain until you kill the ninjas and finish dodging his attacks, use your Earthquake Magic to quickly kill the 2 ninjas, once you dodge his attacks your health drain will go away.
Boss 2/3 -
Remember the orb from the last part of the fight? Well, you'll have to break them again, this time using your Ninja Stars, keep in mind to stay out of the way of the lightning attacks and you'll be fine. Once you break both of them, throw a Ninja Star at him, repeat this process until you move to his next phase.
Ninjas 3/3 -
He will now spawn 3 ninjas with a power upgrade, use your Earthquake Magic to quickly finish them off. Again, you will have to dodge his attacks before moving to the next phase.
Boss 3/3 -
This is VERY simple if you follow my directions. Firstly break the orb in the front with your sword, then switch to your Ninja Stars and break the left and right orbs. *Make sure you've picked up all the magic potions! If you've done so, you'll be able to use your Earthquake Magic, use it and you'll finish the boss off with just one hit (provided you have magic upgraded a little). If it does not kill him the first time, simply do this again and he will die.
Good job, you've finished all the levels! If you need to level up more, continue fighting the boss 6-8 more times until you reach your desired level. (Easy way to level up for the upgrades achievement)