Illidan vs Cenarius

So for some reason I just can not beat Cenarius, I have tried over and over, I don’t know if it is I am kinda of a new player but I get him down to like 16 or 14 health while I am on like 6, I don’t know if there is a certain way to beat this or am i just getting really bad RNG, If anyone could give me some pointers that would be great!