Leedmees

This can be done on many different stages, but stage 11 is probably one of the easiest early stages to try it on.

At the start of the stage, put both hands up with your elbows bent at 90 degree angles (like a field goal on a football field). Wait under the blue portal until five Leemees has spawned (or, you can wait for all to spawn to hopefully net an S rank). Since you have your arms positioned this way, they will be trapped between your arms, continuously walking back and forth. .

Now carefully walk around the stage and pass over each of the stars. Once you have all 5 stars collected, make your way towards the red portal.

At this point, all you have to do is run straight into the portal so every one of the Leedmees enter at virtually the same time.

When I got the achievement, I didn't exactly get all the Leedmees "simultaneously", but the game seems to give you a little room for error.

Here's a video guide of Johnny Sinister's solution. Although the achievement doesn't 'pop' in the video it's exactly how I did it during my initial run through of the game.