From the main area (the area you spawn in when you start the game), go over to the right area where you can see the floating play button with "PLAY" above it.
When you get close enough to the play button, it will disappear and three options should appear. The middle one is the "DAILY CHALLENGE". Aim at the "PLAY" button underneath "DAILY CHALLENGE" and click. It should present the current date's daily challenge. Click on "PLAY" to begin the daily challenge.
You will spawn into the starting room of the arena. Simply play the challenge as you would any other level in the game. Do this 50 times to unlock the achievement.
NOTE: You do not need to complete 100% of the daily challenge in order to get the achievement. Just play the challenge 50 times and the achievement should unlock. Also, you can play the same daily challenge over and over, if you wish. The achievement does not require 50 unique daily challenges.