33 – Route 40 to 41
Head to the western exit of Olivine City. From there, head down on Route 40. Make sure you have a Pokemon that knows Surf and everyone is healed up. The person you will notice by the water just comments on the waves and how no one can resist them. Time to hop on your Pokemon and start toward Cianwood.
After you hop on the water, you will see a swimmer nearby closer to the center of the passage. This is Swimmer Simon. He starts with a Tentacool lvl. 20. It will likely start attacking with Acid, which will follow with Bubblebeam or Toxic Spikes. Toxic Spikes present an entry hazard. After these are laid down, any Pokemon entering the battle will be Poisoned so think carefully about any switches after these are laid (though any Pokemon that is Poison type will absorb these Spikes removing them). After you take down his first Tentacool, he will send in a second Tentacool that is also lvl. 20. It will likely also put down Toxic Spikes, which will increase the amount of damage from the Poison, and probably follow it with Acid or Bubblebeam. In either case, Electric or Psychic attacks are going to result in the quickest win.
Head downwards from Simon and there will be another Swimmer on the right hand side of the route. This Challenger is Swimmer Elaine, who will lead with a Staryu lvl. 21. It may start with Rapid Spin, which is most useful for clearing away entry hazards (like the previously mentioned Toxic Spikes). After that it may attack with Water Gun, repeatedly. It may try to use Camouflage, which will change its type based on its surroundings, but Staryu is Water to begin with.
After you beat her, take a moment to head to the right to reach the small island with an item ball. This item ball has TM 88, Pluck, inside it. Pluck is a move that has your Pokemon steal the berry that your Opponent's Pokemon is holding. Hop back on your Surfing Pokemon and head left. You will see another Swimmer waiting to challenge you, this is Swimmer Randall. He will lead with a Shellder lvl. 18. It will likely lead with a few Tackles. After that, it may use Icicle Spear, a multi-hit Ice attack. It may use Supersonic after that to confuse your Pokemon. He will send in either Shellder lvl. 18 or Wartortle lvl. 20. The second Shellder at lvl. 18 acts a little different from the first, opening with Withdraw, boosting its Defense first and likely following that with Supersonic, again trying to confuse your Pokemon. It may then use Icicle Spear. The Wartortle will likely start with Tail Whip, lowering your Pokemon's Defense, and may follow it with an attack, if it survives long enough.
Head down a little bit to take on the moving swimmer just below Randall. This is Swimmer Paula who will lead with a Staryu lvl. 19. It will likely start attacking with Water Gun and may follow it with Rapid Spin. It will also likely try to use Camouflage, which will fail as mentioned earlier. It does have Recover, so it can regenerate some HP, though it will not really use this one much. After the Staryu, Paula will send in a Shellder lvl. 19. It will have much the same move set as those you just encountered. It will boost its Defense with Withdraw, defend itself with Protect or attack with Icicle Spear.
After Paula, you will reach Route 41