There are really good decks out there that you can will battle. You need to make sure you are prepared for all of them. Here are some popular cards they are based off of:
-Golurk Double-Type (Ancient Origins)
-M Tyranitar + Hoopa EX (both Ancient Origins)
-M Sceptile (Ancient Origins)
-Machamp EX + Ariados (Both Ancient Origins)
-Primal Double (Primal Clash/Ancient Origins)
-M Rayquaza Colorless + Sky Field (Roaring Skies/Ancient Origins) (Roaring Skies)
There will be many more powerful pokemon out there that you need to think of.
If you need more help, read some articles in the following website:
http://www.pokemon.com/us/pokemon-tcg/
-Golurk Double-Type (Ancient Origins)
-M Tyranitar + Hoopa EX (both Ancient Origins)
-M Sceptile (Ancient Origins)
-Machamp EX + Ariados (Both Ancient Origins)
-Primal Double (Primal Clash/Ancient Origins)
-M Rayquaza Colorless + Sky Field (Roaring Skies/Ancient Origins) (Roaring Skies)
There will be many more powerful pokemon out there that you need to think of.
If you need more help, read some articles in the following website:
http://www.pokemon.com/us/pokemon-tcg/