Raskulls

There is another way to do this achievement.

Simply connect 4 controllers and take the fire power with your raskull, arrange the other 3 characters (put them all together) and fire them.

(this method is great with you are doing "tell friends about this game" achievement)

This is easy to do, you just need to find the right spot. This is kind of difficult to describe, but I'll give it a go and include a video of what I'm talking about below as well, just in case you get completely lost.

What you need to do is get one of the flame chargers, then keep moving until you find one of the spots where you need to punch a coloured block to make some skull faced blocks fall onto some other skull faced blocks, to clear the way. If you don't touch the video, the preview picture of the video shows one of these areas. You want one of these spots because while the blocks are preparing to fall, all the other raskulls will just stand still waiting for them to fall, as close as they can get to the skull blocks without being hit by them. Since they should all be staying on top of each other, this makes them all an easy target.

I suggest going for it in the first Grand Prix, there are plenty of moments like that in it. I would also recommend normal/medium AI, easy AI will likely lag behind.

Below I've included a video which will give you a visual guide to this, which is probably easier to comprehend. Credit goes to Achievement Hunter for the video, which will also show you how to get the Smarty Pants achievement.