I've had a few people DM me about this achievement. It's fairly simple - if a little grindy - to unlock. This is how I did it.
First of all - ALL modes count towards this total. Master League, Be A Legend, Online, MyClub - they all count. If you're playing this game for the achievements, doing everything else beforehand will seriously cut into this grind. Once everything is unlocked, do the following:
1. Create a Custom League (Konami) on Beginner Difficulty. Pick a strong team for yourself (I used Man Blue), and loads of lower rated teams as your opposition. Select the lowest time for each half.
2. Play and win every game. This mode only has loading screens to contend with in between games, and cuts down the play time considerably. If you're anything like me, you'll score 3-4 goals, sit in your own half, and watch Netflix on a laptop to while out the time. The beginner COM will rarely score against you anyway.
The achievement will pop on the final whistle of your 100th game.