I find it extremely odd that I struggled on Adrienn the first two times, but third time I just used minimize acid armor muk with 2 Sp Def. X-Items and it 1v6'd his entire team. (Had to let granbull kill Mawile with eq cause I only had Gunk Shot)
If I had to say? Probably Ampharos. I'm REALLY waiting for Amphrosite to be released, for now I'm just using mega Banette. Though I wish Reborn kept Ampharos' shiny the same :c