Panda Punch

The first boss level will be after completing level 16.
This achievement is currently trivial to earn due to one reason: at any point during the boss battle, if you lose one or two lives, you can pause the game and reload the checkpoint. This will respawn you with full health, and the boss will retain its current health (all damage taken).
Keep spamming your attack button on the boss. After taking one or two hits, reload the checkpoint. Rinse and repeat as needed until boss is defeated.

Simply put, spam on the boss and replenish health by reloading the checkpoint from the pause menu.

I find it doubtful that this will change with an update. However, this worked for me at the time of posting.
Please leave feedback if providing a negative vote.

if you need quick to defeat that boss,i made a video,in shorts,

just hit & jump in timing (only around 30 seconds you can defeat it)



https://youtube.com/shorts/aD-wRs7U7Yg?feature=share