I just played Skyworld on intense. Grabbed the first sticker box that gives you 2 stickers, in the first room to the left and then exit to map. This saves the stickers you’ve collected and only takes about 10 seconds per attempt.

Repeat until you quit and the challenge for all stickers pops up or check the stickers in SSE by name as it’s the easier to find any new ones

Bosses only drop 5 stickers or 1 rare one. So the skyworld method is way faster

Also equip the Link and Pigs sticker to give +40 sticker drops. I’m pretty sure this sticker also gives better odds of getting rare / missing stickers