Okay Ken we had a bad break with that freeze, but we can still get him. Bury him in rocks, power yourself up and then do it again.
Rock Slide > Swords Dance > Rock Slide
Fun fact: ASB Ho-oh, Groudon, Kyogre, Dialga, Palkia, Reshiram and Zekrom have more stars than Arceus. Mewtwo is one of the more average legends, having less stars than the fairies.