In order to find an Egg you must be in an online multiplayer match (NOT a private match). When you complete the multiplayer map Eggs you will get 2,000 XP. Then go to Extinction and destroy a hive on Nightfall and you will get the Achievement Egg-Stra XP! for 20G.
After you shoot or knife the egg you will get this little pop up to show your progress.
After you destroy the Hive you will get a bonus 10,000 XP
