At first just destroy the Munge Dweevils and the five fire geysers. On the other side of the sublevel, you will meet a Raging Long Legs. To get the pink flower, it is important to be patient in defeating the boss: it is recommended that you use only five to ten Pikmin. Make good use of the sprays as you attack. Throw your Pikmin onto the body straight after whistling them away each time before they are thrown off because they will be safer there than on the ground. It may take a while, but you still will have enough time to gather The Key and some of the other treasures lying around.