The guide doesn't mention it, but you can actually get all the 1st place achievements by yourself. Just start the game type you want, then press start, and select return to lobby. If you were the only person in the private room then you'l automatically get the win and unlock the achievement. This works for all the 1st place achievments and the champion achievements except for steal the sample, predator, and seek and secure. For those 3 you need to set the match to 2 minutes then run and do wahtaever task is required. The game ends when you've secured the spot (for example) then just repeat 49 more times.
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