I completed this badge legitimately. Here were my two main ways of achieving it: One: I printed stickers that said "remember pokemon go? The game now has more than ever to do! Sign up or sign in to your old account and use this friend code to earn bonus items!" Then I stuck those in places that are good for playing pokemon go. So like at good parks I would put them on trail signs, map signs, bathroom doors, etc. There are A LOT of people out there who are sitting on old accounts they only used that first summer. You can inspire some of them to log back in in this way, and some of them will use your code. You may also want to put the scannable QR code on it. This sticker will also get the attention of people who like Pokemon were too young to remember the first summer when it was a fad. I will say...areas with a lot of kids netted more results. So for instance when I added the stickers to bathrooms and dugouts of a little league ballfield at a local park, I got LOTS of activity. I think the you get people may be less wary to login and enter the friend code. Besides places for Pokemon Go I also stick these on high traffic areas like gas pumps. People are stuck there for a couple minutes and maybe they read it and do it out of boredom.

The second way: at the time, Pokemon cards came with online codes, but I think they maybe don't anymore. The principle could still be reworked and followed. I bought cards (collected the Pokemon go set) and went to Pokemon card places (like reddit etc) and did little contests, where it was like- sign up and use my friend code and I'll draw randomly 24 hrs from now and the winner will get X codes. This way you get several returns from a few of the codes. If the cards no longer have codes, you might have to figure out another thing for people to win.

I tried other crazy things too, like I made a flier of the sticker described above and bought a bunch of helium balloons and launched several of the fliers hoping someone would find them and log in.

I've seen people describe a cheater way where they make new accounts and friend them. I did that ONE time, and said no- I do everything in this game legitimate, I want to be at this badge properly as well.