Navigate to challenge mode, and go to part 6. There you will find the "Racing Day Is A Very Dangerous Day" challenge. Select the challenge, and then select parts that give you the best top speed and steering. After loading into the match, get your boost from the start, and collect the slime tokens directly in front of you until you have 10, then ram into the nearest wall. You will fail the challenge, but be given 20 slime tokens regardless. Repeat this process over and over for easy slime tokens and achievements. I hope I could help! :)