Question: I'm a very new player to the Pokemon TCG, and have picked up two themed decks to play with ([Ultra Prism](https://www.pokemon.com/us/pokemon-tcg/sun-moon-ultra-prism/theme-decks/)).

So far in my experience however it seems that if one player gets their active Pokemon to its final evolution (for example [Empoleon](https://www.pokemon.com/us/pokemon-tcg/pokemon-cards/sm-series/sm5/34/)) before the other player, then that player has a massive advantage, and is able to one shot all of the opponent's Pokemon before they can do enough damage to knock out the evolved Pokemon.

Is there a strategy for countering this? It can turn a game very one sided very quickly.

Answer: 
Which is one of the main concepts to win a game, prepare your pokemon so that they can do massive damage and dominate the scenario (either by evolving it or get it ready with necessary energy cards). 

Since we are talking about theme decks here, cards and scenarios are limited which is really beginner-friendly. Each expansion (in your case, Ultra Prism) comes with 2 theme decks normally (which they balance out the odds of winning between them so they both will sell), each has it's strenghts and ways to take over the game. 

There are a few ways to "comeback" in the game playing any mode, unfortunately (and fortunately at the same time) in Theme mode ways to win is limited to setting up your Over Powered evolutions and swiping your oponents actives. One way to comeback in themes which I have noticed is sacraficing some prizes and Pokemon until you prepare a Pokemon that could take out your oponents active. 

Basically, while your opponent got their active evolved and ready with energies and is swiping your actives and taking prizes (he is 1-hit KOing your active Pokemon), he/she is probably "over comitting" on his active Pokemon, which they have spent a few turns only attaching enery cards to that pokemon and probably do not have other Pokemon prepared on their bench. 
You can take the opportunity to stall by giving him the turns to KO your some of your Pokemon while preparing 1 Pokemon to fight against his "OP" Empoleon. E.g. have your garchomp set with a Cynthia in hand so you can KO his Empoloeon with 200 dmg.

Try to get the cards you need to set up powerful attackers while your oponent is putting all his eggs in one basket, by optimizing usage of trainers and supportes you have in hand.

You need to calculate your sacrafices and chances, and give your best shot. There have been many times where games were turned from 6-1 prizes to 0-1.

I'm no pro player but hope that helps : )
Other answers provide good strategies. I will try to provide additional info and some other strategies and cards that can help with that.

1. As [@TomYumGuy wrote](https://boardgames.stackexchange.com/a/48534/22542) you could **sacrifice** some pokemon while preparing your defeater on bench. There are trainer cards that can help with this like [Raihan](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/ss-series/swsh7/152/):

> 
> You can play this card only if any of your Pokémon were Knocked Out
> during your opponent’s last turn. Attach a basic Energy card from your
> discard pile to 1 of your Pokémon. If you do, search your deck for a
> card and put it into your hand. Then, shuffle your deck.
> 
> 
> 

2. One more option is to buy time with **pokemon/attack that prevents future damage** to it, for example [Shelgon](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/ss-series/swsh7/108/) - Hard Roll:

> 
> Flip a coin. If heads, during your opponent’s next turn, prevent all
> damage from and effects of attacks done to this Pokémon.
> 
> 
> 

[Diglett](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/ss-series/swsh6/76/) - Dig or [Pikachu](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/xy-series/xy8/48/) - Agility.

3. There are some pokemon/attacks with advantage against evolution pokemon like [Hitmonchan](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/ss-series/swsh7/81/) - Clean Hit:

> 
> If your opponent’s Active Pokémon is an Evolution Pokémon, this attack
> does 50 more damage.
> 
> 
> 

4. It is also possible to **devolve the pokemon**, f.e. [Golurk](https://www.pokemon.com/uk/pokemon-tcg/pokemon-cards/bw-series/bw6/59/) - Devolution Punch:

> 
> Devolve the Defending Pokémon and put the highest Stage Evolution card
> on it into your opponent's hand.
> 
> 
> 

Devolved pokemon might not have enough HP left and be knocked out!