Summarize:

Stationed at the highest middle level of the village, an NPC named Hoffman will be waiting for you. Talk to him regarding your class-advancement change and then choose which class you're going to take. Choose Battle Magician; he will then give you a set of tasks to complete. Enter the Suburbs of Wally's Castle under Very Hard mode to hunt for this item.  The strategy for this is to go through the dungeon with a party, preferably your friends. Rush to the last floor where you get to fight the boss, William, as he's the only monster that drops the required quest item. If you're unable to get the pendant, you have to repeat the raid until you succeed. Skipping other floors will save you time, but you will miss mini bosses and useful loots. For the next leg of the quest, you're required to kill monsters in the dungeon of Elder Village. This includes the final boss, Wally No.8.  You must enter Wally’s Castle under Very Hard mode. Wally's Castle has a lot of floors, so make sure to enter each of them to find mini bosses. These mini bosses drop rare items that you'll find useful in survival and a good source of gold if you plan on selling it to other players. It is recommended to enter this dungeon with a party, then enter each floor to clear it of all monsters before heading for the final boss. There is no time limit for this quest. After completing the quest, return to Elder Village to hand your quest item to Hoffman, or simply open your Quest Log and click Complete Quest.  Your character will instantly change its class to Battle Magician after giving the quest item to Hoffman or clicking on Complete Quest in your Quest Log. Take note that there's a small bug: if your character didn't change its class despite of completing and handing in the quest item, log off then log in again to fix this.
Meet the Village Merchant. Obtain a Magic Pendant. Complete Wally’s Castle. Return to Elder Village.