If you want to complete the Meganium’s Pal request you will need a 4-star pose snap and for that, you will need an Illumina Orb.
How To Get Illumina Orbs
You will have to take a snap of Crystalbloom in order to get Illumina Orbs. In Belusylva, go to Founja Jungle (Night). At the end of the jungle, you will find yellow sparkling Crystalbloom on your right-hand side. Complete the Jungle (Night) level and present Professor Mirror with the Crystalbloom picture. He will then give you Illumina Orbs.
How To Complete Meganium’s Pal Request In New Pokémon Snap
In Florio Nature Park, go to Illumina Spot, where the Illumina Meganium can be found. To wake up Shaymin, throw the Illumina orb on the cliff on the left side. Shaymin will hop up on the head of Meganium.
Now, throw Illumina Orb on the Crystalbloom before Meganium sniffs it. Meganium will retreat to the back of the tree after sniffing it. The request can’t be completed if Meganium doesn’t go all the way to the bottom of the tree.
Start throwing Fluffruit on the tree once Meganium is below the tree, so Hoothoot falls on the top of Meganium and surprises Meganium. Take a photo of Meganium in a shocked sitting state. This is a 4-star pose snap which is required in order to complete the submission.