I am assuming you have completed all the Player vs. AI trainer challenges and unlocked all the free cards there... (Beat each Trainer 4x, max level each theme deck to 7/7 to unlock new cards and boosters, complete all three tiers of trainers, etc.)
Be sure to do your daily challenge (click your little level icon) and your Versus ladder dailies in the beginning (set it to theme or standard to start, not expanded/unlimited) to ****** tons of better cards. Once you have 500 tokens go to the shop and grab up a new theme deck (good ones are Mach Strike - Garchomp, Hidden Moon - Lunala, Imperal Command - Empoleon, Gears of Fire - Volcanion, Blazing Volcano - Entei, Battle ruler - Zygarde, Torrential Cannon - Blastoise, Relentless Flame - Charizard; in no particular order, whichever you prefer) and then max those to get more boosters - just to the 12/12 mark and unlock them for theme deck play in VS.
Keep switching decks and keep beating trainers you haven't with each deck until you unlock all these things. When you run out, hit VS and do more theme deck battles. This will also give you a ton of strategy ideas with decks you are already familiar with from the Trainer battles and there is no real chance of getting ******** on the first few turns as the opponent is playing on a fixed system.
Just a final point...