I have never participated in one of these before and I want to try it out so I am trying to figure out most of the common threats I am gonna run into so I can make sure I have at least something to deal with them.

Obviously Mewtwo, Xerneas, and Yveltal are gonna be everywhere, but I imagine I will see plenty of Talonflame, Rotom W, Azumarill, Mega Kangha, Mega Charizard, Mega Mawile, Mega Gengar, Greninja, and Aegislash, to list a few off the top of my head. Are there any other common threats I should keep an eye on and to those who have done these before do you have any advice?