There are two ways that you can go about getting this simple but tedious achievement. You can play the game legitimately and get this on your way to rank 50, which will take much longer than 100 hours.
However, if you want the set of time played achievements quickly, the method that you can use for each of the play 10 hours as the three classes achievements works here as well. When you finish a level, rather than go through the door and seeing your rewards from the level, just let it sit. The timer will have stopped at this point, but the game will continue counting the time spent for these achievements. Do this for just over four days and you will have "played" for over 100 hours.
Note: You must finish the level for this time to count. You won't make any progress if you do not go through the final door after you are done idling.