Walk straight up and pick up the Herb first, then the Shield and Staff in that order. Equip the Shield. Head to the entrance of this room, leaving the Thiefwalrus alone. Before you enter the hallway, throw the Herb across the ground. In the Hallway is a Pitfall Trap that you need to avoid, so move away from the entrance and let the Thiefwalrus chase after the Item.
Once it gets across the Traps (3 tiles in the hallway.) You should switch with it using your Switching Staff. Next, throw the Herb across the water so it lands anywhere on the island near the Stairs. Head down and into the last room to pick up a Knockback Staff. Return to the room with the Stairs and the Thiefwalrus will be trying to get to the Herb. Simply use the Knockback Staff to hit him diagonally across the water. Then line up with the Thiefwalrus and switch with it again to reach the Stairs.
