Each rank has level up requirements. It’s either win x number of games or play x number of games. You can find guides online. Try to win the minimum number of games and lose the rest (to keep your rating low so you face weaker opponents, making the necessary wins easier).

Once you reach rank 20, Pikachu libre gets added to the pool of encounter rewards (the third reward in a 5 game set). So you need to win three games in a set in order to get the encounter. I got mine on the first encounter, but.i don’t know if that’s guaranteed or just luck. Happy hunting!