Invasion mode is 5 levels of 3 waves, each level gets harder as you go. Enemies vary from Trust guards to Drudge drones and Scarabs and Jade Warriors to Advancers in full heavy armour.
Good to get aiming corrected and one of the best way to earn quick credits.
As to where to find it, look on main menu once you open a profile, normally above Online Multiplayer and under Continue Game.
Plus completing each level of Invasion earns you yet more Achievements. --- NNID - AndyInTheUK Mainly playing Zombi U. Now number 3 on the Survival mode Leaderboard for Zombi U. http://i.imgur.com/YD02k.jpg