I cleared the challenge!
I'll list the pokemon and how usefull they were in the league.(With comments)
Kingler 5% Tentacruel was better for water moves, while Marowak was better with Ground moves. Leaves it's only usefull move Vicegrip.
Tentacruel 40% He KOed a lot of pokemon, while he SOLOed 3 of my rivals pokemon!
Marowak 5% He was too slow to launch many attacks, while he oftenjgot 1-hit-KOed.
Tangela 10% He wasn't really useful, but at least he KOed Lance's Gyarados, who normally gives me lot of problems.
Beedrill 10% His stats were too low to really use him in the league, but he pulled me trough the game till the point I could catch Cubone. I only used him in the league after that. I beat Brock only using him, by waiting till Poison sting poisoned and keeping myself healthy with Potions.
Executor 30% He wasn;t really usefull as Execute, but that changed upon evolution! He could KO a lot of opponents while keeping himself healthy with Giga Drain. Psychic also helped a lot!
Too bad Marowak and Kingler didn't reach the level to learn Crabhammer and Bone Rush.