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Locking on to it will be troublesome when you miss some orbs since the camera will follow it. It will start dropping some orbs, but with a lot of draws equipped, you won't have to move much yet. Do this when the mushroom moves faster and drops a large number of orbs. Luckily, the mushroom's movements are linear, so you'll be able to collect all the orbs by using quick run. Eventually, the mushroom will move faster and you'll have to glide after it. Mushroom no. 3 is one of the hardest mushrooms to beat, so you might have to repeat this a lot before you get it right.
Press ∆ in order to start the challenge. Don't lock onto the mushroom. Keep close to it. Quick run (analog + hold □) to grab the orbs. Glide (analog + □ while in the air) to get closer if you get left behind. Keep repeating these steps until you collect 450 orbs.