In this guide, I’ll help you defeat the Core without taking any hits, I’ll detail the attack patterns and explain how you should react. The Core is the toughest boss during a Hard Mode run and you should be prepared to die multiple times.
THINK FAST WHILE CHANGING WEAPONS, with the Polar Star you will spam the fire button but with the Blade you will be more punctual with your shots, know which weapon you are currently using all the time and know which button to press whenever you change for one of the weapons.
Starting with fragments and luck draws, do you remember when I said that the fragments changes the battle position during waves? this position is random and based on rng, you will have to deal with some position types that I will mention below.
Surrounded
The five fragments will surround you from all directions, in this situation you can dodge by jumping and hitting some of the shots with the blade, if the core is attacking , this will be very risky, I recommend retreating if you want to play it safe.
Horizontally
In this formation the fragments will be aligned horizontally, arguably the best formation to deal with, play aggressively switching between the blade to defend and the polar star to attack, maybe some fragment will be a little above you, in this case it is recommended to jump and shoot it away with the polar star.
Random positions at the top or bottom
It is very easy to dodge jumping at the right moments, if you are not confident there is always the option to shoot with the blade both up and down.
Thanks to DárioSparks for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.