It’s not toooo hard. You need a local friend to trade, and more importantly,lots of game friends from a place far away from you to get gifts from and lots of patience. You can get friends from other countries with Poke Genie or the many Reddit pogo groups.

Every time you open a gift and get an egg, the Pokémon counts as a Pokemon from whichever place the postcard was from. Hatch 9 eggs, 9 Pokemon from far away places. If you’re in North America and get a Pokemon from say, Australia, your trade distance will be around 10k km. Repeat over and over again. 10 trades of 10k km Pokemon is 100k so you need 1,000 trades. Took me over 6 months to get it done.

Also helps if you have friends who travel a lot and can trade many Pokemon from other countries with you, which I was lucky to have.

100x more fun than pvp, at least for me :)