Beat The Boss in a little, but extreme hard RPG

Yesterday the Hacky Easter 2018 Teaser Challenge was released.
You have to play a little Role-playing game that was created with RPG Maker. The goal is to defeat the boss and get an egg with an encrypted key. But defeating the boss is not as easy as you might think. Because the boss is overpowering and cannot be defeated without tricks.

I just beat the boss and got the egg. The following screenshot was taken right after the boss fight, when I got the egg.

Here is the Link to the challenge:
https://www.hacking-lab.com/user/cases/cases.html?event=1389