Ultimate Chicken Horse

This is a cumulative total of all games played, either online or locally, in either Creative, Party, or Challenge mode. Naturally you should get the 50 online games out of the way first to count towards this achievement. Once that's done, there are a bunch of achievements that are specific to games, such as collecting coins, getting solo points, post-mortem points, etc. So as you work on those, you'll make progress towards this achievement.

If you just want this achievement as fast as possible and don't care about efficiency, playing offline with local couch co-op (using the red couch in the lobby to play as 4 people with the same controller) is actually the fastest way to do this, because the game considers all four of those characters to be your profile, so every single game you play actually counts as 4 games toward the counter. You can pause in the lobby and set the game rules to minimum number of rounds (2) and just end games very quickly if you just want to knock out this achievement. But as I said, it will come as you work on all of the other achievements.