So far, I have a Frogadier, a Cherrim, a Loadred, a Swirlix (which I'm not going to use because of the disadvantage), a Litleo, and a Pachirisu. I'm going to try to level up the Mime Jr. I have to switch Swirlix out with for this battle.
Edit: I don't wish to use Gyarados because I already have a Water Starter and I really want this game to be more of a challenge for me, so no OP Gyarados for me. xD