From the Super Mario Wiki
The Spear Thicket is one of the places in Mario Party Advance that is located in the Jungle Area of Shroom City. What makes Spear Thicket unique from other areas is that the player needs to be a specific character and does not have to play for a Gaddget.
Upon stopping, players will meet Spear Guy, a warrior who is looking for a mustached man to challenge. His quest, Mustached Hero!, involves playing as Mario or Luigi. As one of them, they must go to Spear Guy's area, where he will challenge the player to a duel, specifically the Duel minigame Tank-Down. Success will earn the player the right to keep the game.