Impressive. I haven’t managed to do that myself yet.
Well, if you can get past the first phase then what you do is this: you wait on one of the side platforms (I personally prefer the right one), and you wait for him to telegraph its attack right beneath you, if is is doing is stuff where he jumps over the your platform by going from right to left then you need to dodge, in all the other cases stand still, once he does telegraph the attack right beneath you then you jump to another platform and you spam attack until he is sunken again in the slime. Then you repeat until the boss gets to the 3rd phase, there you need to move a little faster and beware the fake telegraphs.
Just know that bosses 1- 4 get trivial with time, and so do 6 and 7, if you are a professional then also boss 8 (first phase) might become trivial, but 5 and 8 (second phase) are going to take you soul multiple times.
KEEP TRYING!!
If you go very aggressive on the "phase" where his tentacles move in a random direction and you attack every single time that it is possible then you can get to the actual phase 2 unharmed and with him only being able to pull 2 attacks of phase one, then it is just timing your jumps and going left from time to time to avoid being hit and you can defeat the boss hitless just like that!!
there is a way to time your jumps so that you can always dodge the fireballs by tapping the opposite direction of the one that you are facing after you begin the jump, also if you are fast enough you can get some hits on him before the screen turns black by jumping off the platforms at the side of the board.