I Ace and collect all of the currently accessible single-player Goodies in the "The Terrible Oni's Volcano" level. Despite the ominous name, the large level badge, the concept of exploring a large level in a vehicle and the fact that this is the last level for this area, this level is actually very linear and easy. If we judge a segment's difficulty by the number of times I had to reattempt the level to get a good recording for Acing and Prize collection, then this level is by far the easiest of the three main "The Islands" levels.
If you really wanted to, you could get all the single-player Goodies in one go by backtracking once you acquire the Bouncy Cloud Sticker a bit before entering the volcano.
I've had some interesting things happen when I've fought The Terrible Oni before. During my recording attempts, I remember a time when the flame-throwing cat was close enough to destroy a bit of the Oni before being knocked just out of range. The Oni ended up missing his upper jaw and not being able to spew fireballs, and the effect was quite creepy to say the least. Another time, the Oni exploded in such a way that his/its knife landed in front of the wobbling platforms and made it very difficult to proceed without dying a few times to push the knife into the pit. Watch out for crazy events like that when fighting The Terrible Oni.
Revisit: After getting the Bouncy Cloud Sticker (that can be found partway through "The Terrible Oni's Volcano" level).
Now that I have the Bouncy Cloud Sticker, I quickly revisit the "The Terrible Oni's Volcano" level to pick up the rewards from the respective Sticker Switch. Then, I cover the Cooperative Challenge (and its exclusive Goodies) for this level.
Special thanks goes to the random online players featured in this video. The male with the Cute Lion Ears goes by the PSN ID of lashams. The pink material female had a display name of Kathleen, but that's not her PSN ID and I don't know what it could be. My guess is that she was probably playing LAN with lashams (as I imagine was the case for Luis in the videos with Luis and Daniel).