The general strategy to get this achievement is to win 100 online matches. It doesn't matter if it's Ranked, Player or KotH matches, wins on all of those modes count toward the achievement.
On a related note, any wins that you get by virtue of the opponent disconnecting or exiting the game before you can pull off your fatality (AKA rage-quitting) do not count towards the achievement. Be very persistent when you're going after Wavenet, as there will be quite a few rage-quitters online.