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Since May 2020 now you can play Casual, so I find that you can "boost" a little to obtain boxes.
What do you need to boost? (I'm level 15, so I'll explain from my point of view, you can escalate to your level and cog tags)
2 accounts (I have my husband's account and my account, he is lvl 14)
2 mobiles phones or 2 PCs or 1 mobile and 1 PC
The same level and cogs tags (that's the perfect setup, most important is the same or +100 or -100 cog tags)
Now that the game had 2 modes for searching matches, I'm not finding matches like the past seasons, I wait more that 10 minutes for finding a match, and I found the same players (with pins 2 or 3 levels higher than me) that I can't beat.
So I found this method a fast way to earn 4 boxes (and if you paid crystals to open up boxes, you can get more boxes obviously). You need to know at what time there will be less players playing, so you can find the 2nd account easily. Just play matches against the 2nd account and win. I recommend that the 2nd account puts pins in the match just in case.
The problem is that the 2nd account had the same cog tags and level than you. Or you can just do a gaming session here and search for a player with the same lvl and cog tags. Now that you can lose on casual without losing cog tags, I can see that this method will work for everybody.
Also, playing horde helps a lot. I have now a very active crew and I won almost 50 golden pins only playing horde, and I still have more than 150 horde thumpers (I have another 150 horde thumpers, but for coin fortune)
Good luck with this achievement if you are a F2P like me