Summarize:

Right after you have completed the Taker chain quest, you will be given three choices for your very first class advancement: Sword Taker, Over Taker, or Weapon Taker. Choose the Sword Taker.  If you make a mistake, you can cancel the quest by simply opening your Quest Log, clicking on the first class advancement quest, and then choosing “Withdraw/Cancel.” The Sword Taker is one of the fastest class in the game. Even the Elsword, who has great speed and attack power, cannot match the Sword Taker's quick movements and speed. Aside from fast movement, the Sword Taker has decent HP; this increases their ability to survive in every battle. After accepting the quest through your Quest Log or taking it from the Elder Village's announcement board, you will be asked to meet Lenphad in Elder Village. You can find him right next to Echo, working as the Blacksmith. Lenphad will give you a set of tasks before he can change you to a Sword Taker. All the quests will be collecting quest items rather than killing monsters. All items can be obtained from one map. To complete this last leg of the quest, you have to enter Wally's Castle under Very Hard difficulty. You must kill Wally's Guardians, Soldiers, and the mini-boss Vergnert in order to obtain the quest items listed at the right-hand side of your screen.  You may enter the dungeon multiple times without losing your progress, and this quest can be done while in a party. There's no time limit or required grade, so it's best to enter every floor for monsters, which may give you rare item drops as well. After you have completed the quest, return to Lenphad and he will turn your character into Sword Taker.
Take the Sword Taker quest. Meet Lenphad. Fight the monsters in Wally's Castle.