Raskulls

Probably one of the easiest achievements in the game, if you have two controllers that is.
Sign in with your profile on one controller and another profile on a different controller. Start up a local game and get the other profile to the end of the "level". MAKE SURE THE RASKULL DOESN"T TOUCH THE CHECKERED FLAGS.
Then get your main profile to the end of the level. Jump on the other raskulls head and then jump to the checkered flags. BAM!! Achievement Unlocked!!

The easiest way to get this is the Snowy Shenanigans level in the Wand Cup. What you need to do is use a Raskull as a stepping stone to reach a finish line you wouldn't otherwise be able to reach.

Single Player --> Quick Race --> Select Raskull --> Select the Wand Cup --> Snowy Shenanigans --> Easy AI

Race towards the final section, and make sure you're the first to get there, which shouldn't be hard at all considering you're fighting easy AI. When you get to the part where you can see the finish line but can't quite reach it by jumping, wait until another raskull shows up. They should work over to the left where the block needs to be destroyed to progress, they should just stand still next to it for a few seconds. While they do so, jump on top of their head and use them as a stepping stone to jump to the finish line, which should unlock this achievement for you.

Here is a video guide as well, in case you are more of a visual person. All credit goes to Achievement Hunter for this guide, which will also show you how to get the Flame Grilled achievement.