Today, I did a bit more training on One Island until Articuno levelled up to lv. 96. Then, I returned to Kanto and went to the Pokemon League again. I sent out Articuno against each of the Elite Four’s Pokemon so that it got experience every time, and switched out to one of my other Pokemon whenever Articuno couldn’t handle whatever it was up against. After that, I went back to the Sevii Islands for more training. I got Articuno to level 99 at Resort Gorgeous, then returned to Seven Island to get it to grow one more level. After a few rematches there, it reached level 100.
Now, I’ve trained my 3 legendary birds to level 100. In the future, I may train my Mewtwo up to lv. 100 as well, and maybe my old Dragonite too.