This achievement requires that you complete 100 full games of Snap Attack. In order for a game to count, you must be present when the round starts and finishes AND play at least one snap. Once you meet the requirements and finish your current round, the achievement will unlock. Each round is 2.5 minutes long, and the post-game lobby lasts about 45 seconds. In total, this means you will need to spend around five hours playing the game to complete 100 rounds. You can check your progress in the stats on the Main Menu. However, the counter for this achievement is separate and is usually several games off the stats counter. If you reach the threshold there and don't unlock the achievement, keep playing as it may take up to 10 or more additional games to pop the achievement.