To quickly grind stickers, you're gonna need some things before you start. You'll want a Kirby, King Dedede, Meta Knight, and Waddle Dee amiibo. Make sure they're specifically from the kirby line of amiibo, they'll have a blue base. These amiibo are not required, but they are recommended because they give you a sticker everytime you scan them.

Secondly, you'll want a lot of play coins. You can buy a random sticker for 3 play coins, and you can do it 5 times before having to beat another level to refill you play coin uses.

What you'll want to do is go to 1-4 and repeatedly replay the level. The level has 4 stickers for you to collect, assuming you've already collected the rare sticker. 1-4 is also a very short and easy level, so you wont have any trouble beating it.

If you're using amiibo then make sure to scan waddle dee, meta knight, and dedede first before kirby. This is because the UFO ability has an attack with spark properties, which is needed to collect a sticker.

If you aren't using amiibo, then make sure to grab the ESP ability thanks to it's spark properties and just generally its a great ability.

Lastly, try to go for 1 at the Goal Game since that can net you an extra sticker. In total, assuming amiibo method, you should finish with either 8 or 9 stickers based on the goal game score.

It's not required, but you can also use play coins to redeem 5 stickers after each level completion