To obtain the last secret technique in Pokemon Let’s Go, you need to head to Fuchsia City. Once you arrive in Fuchsia City, make your way to the Pokemon Center. On the right-hand side of the Pokemon Center, you will find a man with a Lapras standing next to him.
Approach the man and engage in a conversation with him. He will offer to teach your Pikachu or Eevee the secret technique called “Sea Skim.” This technique allows you to travel across bodies of water on Lapras’s back.
Accept the offer and your Pikachu or Eevee will learn the Sea Skim secret technique. From this point on, you can use Sea Skim to explore new areas and reach previously inaccessible locations that require water travel.
It’s important to note that Pikachu or Eevee will be the one learning the secret technique, so if you have both Pikachu and Eevee in your party, you can choose which one learns it. This means you will need to have either Pikachu or Eevee in your party to use the Sea Skim technique.
Once you have learned Sea Skim, you can use it to cross bodies of water and access new areas in your Pokemon Let’s Go adventure.
In summary, to obtain the last secret technique in Pokemon Let’s Go, go to Fuchsia City and talk to the man with a Lapras next to the Pokemon Center. He will teach your Pikachu or Eevee the Sea Skim secret technique, allowing you to travel across bodies of water.