How do I take out Pipes?

Have a frenzy and ram into them.

There are probably other ways but thats how I do this challenge.

So find a course with pipes (the SNES Mario race and Toad's race do have them) and find a character that you've got a 3 items per box with, and then just play it a few times. When you get a frenzy, just go ram some pipes.

Besides frenzy, you can destroy pipes with Bowser's shell (Bowser, Dry Bowser and Bowser Jr. have access to it) or simply get a mega mushroom and ram into a pipe. I've found that I get Bowser's shell way more often than mega mushrooms, but your mileage may vary.