Win at least 100 (non-Custom) online matches. You can see how many you have won thus far via the Profile > Statistics screen in the main menu, or the leaderboards. Both 1 and 3-Lane games are acceptable. You should have everything else unlocked by the time this unlocks; chances are quite a while before this unlocks. Regular and Elite Battlegrounds, as well as Skirmish wins all count toward the 100. Do note that your custom match wins will also be shown in the total wins section of your stats screen, and those do NOT count.
One way to hasten your progress is by going to Settings on the main screen using or and switching your game to Gold Mode, which is near the bottom of the settings. This will remove any DLC maps or guardians from your game, but it will allow you to play against primarily AI, and newbs, for easier kills/wins, and shorter wait times (usually).