As you may not know, I am a huge Popplio supporter. As for a boy named Will, he supports Rowlet. So, I am going to wreck him. Here's my strategy. 

So as you may know, Primarina can learn Blizzard. You you know what I'm going to do? Train Primarina's stats to the max, including Speed. So when I engage in battle, I'm faster. But if he's not faster, he has amazing defense. So I use blizzard and BAM! One shot Decidueye. The deal is the loser has to release their Pokémon and they have to go up in front of everyone and yell

"I am the Ultra Donkey."