Just to add to VileBabas' solution, the achievement says that you have to play with 100 users, but it doesn't mean 100 different players. Every time you play with someone in the Legends mode (even if you played with that player before), it'll add a user to the total, so if you're playing with one other user, that counts as one user, play with two and it counts as two, and if you play with three others, it counts as three.
So, if you happen to do the diving solution that VileBabas suggested with three other players, this could be done in 34 games, whereas if you do it with just one other player, you will have to go through the full 100.