Well, Yunalesca is usually the hardest of the story bosses to beat imo (or maybe Seymour sometimes), so you should probably be fine. Even if you go to Sin, you can still go back if you find out the monsters are too hard.
One of the easier powerboosts you can get is getting some of the stronger Aeons such as Anima or the Magus Sisters. Some of the ultimate weapons actually aren't that difficult to get either.
Oh, it would also be great if Yuna has learned holy, really a great help.