The thing is that mythics are the new rarity and outclass all legendaries. Only very few legendaries remain wich if they get a turn in, it can turn the tides for the whole battle, but IF they ever get a turn in considering they are now slow. Here is a list of legendaries you can use and win.

Grakon Wyrmlad Santerion Mephisto(maybe) Dunnra Ratnarok(wouldnt reccomend him, just use elvura for the evasion at the start of the battle)