This is more a pro-tip than an actual guide if you feel miserably weak yourself in the boss fight, like I did:
I reached this specific boss at character level 19 and barely managed to pull down the her health bar to 50% in my best run. Decided to level up myself. Though +10% attack and +15% defense skill don't seem much, trust me, it can be divide between life and death. Bosses are considerably easier if you 'overlevel' yourself.
I found a very good spot for gaining XP and levelling up in a fast pace:
This spot is early in the game in the first area so you can reach this after the first boss and after obtaining slide ability. For an effective XP grinding, ‘auto-aim’ ability is also recommended but not required.
To grind XP: go to the above spot, which is 4-5 screens right from the teleport station of the first area (you need to have the slide ability to reach the spot) and simply switch the screens (back and forth) and finish off the respawning enemies.
The screen on the right includes weak ‘blue flame’ enemies (2 are hiding in the penguin statues). One single shot + one auto aim shot will clear the screen. (Be close to the guys when releasing auto aim and your HP and MP will be filled back.)
On the left screen, there are 2 penguins and a cat. Cleaning the two screens grants 19 XP and can be done in 10 seconds. At level 20-25, the level-up requirement is at around 400-450 XP. You can easily grind this amount here within few minutes.
I went back to the boss with a level 30 character and beat her at first attempt.