Dollar Dash

The easiest way to do this is to create a private match (any game will do) with three guests and adjust the powerups so that only boulders are available. Move everyone else in a line and crush them with a single boulder.

If you don't have enough controllers, you'll need to create a boosting session on TA.

NOTE: You'll want controllers with an actual LS to do this. Also, you can't use guitar controllers. I thought I could get away with using two guitar controllers on Arena (I could move the third character between the two guitar-controlled characters to make a line), but the guitar controllers apparently send a continuous stream of button presses to the game that cause a dialog box to endlessly pop up.

Boulder Dash - 20G (Original Achievement)

Difficulty: Very Easy

Estimated Period of Time: Very Short
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To get this Achievement, you must roll over 3 "human" players with one boulder.

These 3 can be either boosted on the site, guest accounts on the same console via other controllers, or a combination of the two.

Create a Private Match (Which mode doesn't matter, I used Save the Safe)

Once you have all four players (including yourself), load up a map like Skyscraper, turn off all weapons except the Boulder, and load the match.

Have the three others line up, then mow them down with the boulder power up to get your Achievement. Just make sure you return the favor to any other player, be a sport!