Online PlayerG 20

This achievement requires you to host or join an online game through completion of a level. You can join other games from the main menu > Xbox Live, or host games in the Pause menu > Xbox Live Settings.

To make this super easy you can have someone join your game towards the end of a level and complete it. That will unlock the achievement for both parties.

If you're looking for this achievement and need someone to help you with this, feel free to send me a message and I'll jump on with you.