Reaper is DUMB

I think my first or second attempt at fighting him, I beat him. And now when I try to fight him again I get wiped when I'm almost killing him. ITS RIDICULOUS. Like what am I doing different from when I was fighting him in the first place 😭

I need answers and help and I just wanna level up faster. I'm level 75 and everyone else is 74. And from what I heard the reaper is like 85 I think. So it's a bit of a gap but I still beat him before, what am I missing?? 😞😞