Instructions
Capture a Clamperl at Route 219 and 221 using a "Super Rod." Fly to the Battle Zone's "Fight Area," located on the island northeast of the "Sinnoh" region mainland. Walk inside the lookout station bridging the main Fight Area to Route 230; talk to one of the fishermen to receive the Super Rod. Head back to Route 219 or 221 and use the Super Rod to capture Clamperl. Use primarily Grass and Electric Pokemon to weaken Clamperl, because of its type disadvantage as a Water Pokemon.
Train your Clamperl to your desired level by battling wild Pokemon or Pokemon trainers with the help of your "Vs. Seeker," a "Key Item" used to re-battle trainers. Use "Technical Machines" (TMs) to teach your Clamperl useful moves; some moves of interest include "Water Pulse," "Brine," "Ice Beam," "Blizzard" and "Protect."
Find the item "Deep Sea Tooth" from wild "Carvanha" and "Sharpedo." Pastoria City's Safari Zone "Great Marsh" area houses Wild Carvanha, while Wild Sharpedo reside within the waters of Route 213 and 222; use the Super Rod to catch either Pokemon. Battle a wild Carvanha or Sharpedo using a Pokemon with the "Frisk" ability. Frisk allows your Pokemon to "check" a wild Pokemon's held item without capturing it. Common Pokemon with this ability include "Banette," "Shuppet" and "Stantler."
Negotiate a trade with another player to evolve your Clamperl into a Huntail. Head upstairs to trade your local wireless connection or downstairs to trade via Wi-Fi. Save your game when prompted and head into the local wireless hub; talk to your trading partner and select "trade" if trading via wireless. Select your player from the Nintendo Wi-Fi Connection menu if trading through Wi-Fi; choose the "trade" option after selecting your trade partner's name and icon. Perform your trade and watch your Clamperl evolve into Huntail after the trade completes. Trade back your partner's Pokemon to receive your Huntail.