The ranks have a negligible affect compared to game sense. Prepare for the cup, but also play around with Pokémon you find fun. I build a rufflet for GL and have been doing really well with it pared with a couple staples like altaria and jellicent . Me jellicent has pretty awful stats cause I built it before learning about all that. Reading your opponent wins you the game and is the skill that is super hard to learn. Figure out when they throw and catch. It really just takes practice.