Acerola is the Ghost type Elite 4 member in the games Sun, Moon, Ultra Sun & Ultra Moon. With Acerola's Sun & Moon team being Sableye, Drifblim, Dhelmise, Froslass & Palossand with Ghostium Z, so for the Sun & Moon battle against Acerola, my suggestion would be to bring a Fairy type solely for the Sableye, since that's only type that can deal super effective damage to Sableye, or just something with a Fairy type move would work as well. Then also bring a Dark type, since Dark resists Ghost, or you could bring your own Ghost type, would just be a little more risky. I'd say best pokemon to beat Acerola in Sun & Moon would be Mimikyu, as Ghost/Fairy dual stab can easily take down her Sun & Moon team. Then in Ultra Sun & Ultra Moon, Acerola's team is basically the same, only difference is she uses a Banette over a Sableye, so here you don't need a Fairy type, just basically a Ghost or Dark type can work against her USUM team. Mimikyu is probably against the best one to use, just because with Mimikyu's disguise ability, can take a hit with no damage essentially. I really love Acerola's design, and I can proudly say Acerola is my favorite character design within the Alola region, and her design is more why I love her, but her teams are also pretty good too, and those teams definitely don't make the battles against Acerola an easy match. I was originally planning on having Acerola in 3rd, with Caitlin 2nd and Lorelei 1st, but I opted to put Acerola above Caitlin & Lorelei just because I felt Acerola was the most well-rounded member of my top 3, with having a great design alongside great teams. With Caitlin lacking a little in design, and Lorelei lacking a little in team, then I considered Acerola being middle of the pact for both categories, and that is ultimately why I decided on this result. Next ranking will be for the Pokemon Champions, which will be coming to you in the upcoming weeks, so stay tuned.