I was just thinking this same thing. Some are grind-ier than others, but all the successful ones boil down to what you said.
I used a 4* Trooper to get past the general, but I beat Beezelbub with all 3* units. Now on Underworld I can 4* finally. I was underpowered for the General, I do believe that I could beat him now. It really is just about putting in the time (or getting lucky).