Live Duels are unlocked on level 20.
As usual, you’ll be able to pick one monster from three different possibilities. Right after you choose your first team member, you’ll be offered three more. And three more after that. So in the end you’ll have your 3-monster team with your 3 picks. Lots of threes, indeed. But we know you got it ;)

(Almost*) All these monsters will be Mythics, so bye-bye Legendaries, you’ve been great partners, but Mythics have arrived like a tornado and they’re here to fight! It’s important to say that there will be no talents, just monsters that are eager to win!
Let’s talk about the particular Duel Modes 1 by 1.