Check out the awesome challenges guide here from
Segendary Ty:
In particular, this spreadsheet shows you the most efficient order for doing the challenges:
https://docs.google.com/spreadsheets/d/14g4-Z9Osry2Agiz334Yj...(there's a different tab for each number of players)
There are 50 freeburn challenges each for 2, 3, 4, 5, 6, 7 and 8 players, making a total of 350, but you only need 25 of them. The easiest way is to grab a friend (or start a session), and go for the 2-player challenges. Remember you only need half of them, so feel free to skip ones you're finding tricky (unless you know you'll eventually want the achievement for scoring all 250, linked above).