In order to wave at players you bring up the emote menu by holding
+
together, and select wave by pressing
on the D-Pad. As the achievement description suggests, it must be done to an enemy who is no more than 5 meters away from you and they must also be waving back to you simultaneously.
Best method that worked for me was to join a random player's game and join their faction (either automatically if their settings allow it, or wait for them to approve you manually). Once you're in you can then spawn at their base and communicate with them that you'd like to knock out this achievement and see if they'd be willing to do it with you. If yes, you can then go ahead and leave the faction and do the required steps above to pop the achievement. If you have a friend playing this game as well this is even easier to boost. Hope this helps :)