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I heard you have a lot of luck for these trades on discord. Do be careful regarding scams, check their profile and see if they have some referrals regarding that.
Sorry for the double post, but thought this might help you see it faster. I just got my Zamazenta (I had Zacian), through Pokemon HOME. How I did it:
1. I checked if anyone was trading a Zamazenta
2. A few people were and almost all of them wanted a FurFrou. Of course I don't have a Furfrou
3. I checked who is giving away a Furfrou and what pokemon they want. There was one guy that wanted a Shedinja
4. Sent my Shedinja for a Furfrou
5. Tried to send my Furfrou for a Zamazenta (didn't work, they only wanted some special type)
6. Put my Furfrou up for trade on the GTS for a Zamazenta.
7. 5 minutes later, I got my legendary Doggo.