Rafflasher is an enormous plant deep in the marshlands.
It has two methods of attack. First, it will spit out blue seed pods that will hover in the area and lash out with small tentacles. These airborne seeds will eventually self-destruct, but Rafflasher can have a couple out at a time, and they'll inhibit the Master from being able to jump safely. The other method is that of a vine that prowls beneath the swampy surface. It tracks the Master's movements, and thrashes about when it's close. This vine cannot be destroyed or even slowed down.
This is a vicious fight. The Master must lure away the vine by moving away from Rafflasher, then dodge past the vine and land a few hits before it catches up. Then, repeat the process: lure the vine away towards the entrance, then dash to the plant's blue bud of a body, attack as swiftly as possible, and lure the vine away again. All the while, the Master must avoid or destroy the blue seed pods. Using Magic is not really recommended, as the damage to Rafflasher will be minimal and it'll give the vine time to track and attack the Master.
Rafflasher's name is based off "Rafflesia", a genus of parasitic, flowering plants, which actually describes the boss fairly well. Rafflesia flowers can be seen in the background during the fight.