only phase 2 and 3 cuz 1 is ez
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!!