Big Bumpin

This is very easy to do by yourself with just one controller. Move to Ice Box Last Man Standing and customize it by hitting X. Make sure the AI is easy, pickups are off, and hazards are off. Health doesn't matter, but I jacked it to 1000 anyway.

When the match starts, the AI will all ram each other and cluster near the middle. I just circled them for a bit and they started to come after me, making a little conga line spaced out on my tail. Then I drove in a big circle with them following for 30 seconds for the achievement. With no powerups or hazards to interfere, they shouldn't be able to catch or harm you.

Easy!

If you have multiple controllers, easier than boosting online.
Simply start a 4 player multiplayer game on any Last Man Standing stage,
wait thirty seconds and pop goes the achievement.
As a note, I only had 3 controllers and used the controllers not going for the ach. to knock out the CPU character at the start, I was using the 'Broiler' stage so that I could knock him off the map immediately.

This achievement is easiest online while boosting with other people. Set the game type to 'last man standing', and have everyone just sit there and you'll unlock this achievement.