This solution will work perfectly for the Challenger and Challenge King achievements.
While online on the Community Challenges section simply pick a challenge with a red cog in the left corner (you can pick one with a tree or lightbulb but those are not needed for the achievement)
Simply complete 20 of these and the achievement will be yours. It is as simple as that.
Note: You cannot repeat the same challenge 20 times, I tried and it dosen't work.