To start double feature once you have completed the EE start up the map and press the
on the button across from the generator.
I'm going to start off by saying this is going to be tough, no matter how you look at it. To unlock this trophy you have to have access to "Double Feature" which is only unlocked after completing the Easter Egg. THEN, you have to beat round 20 (second Oz fight.) You can play with a friend if they have it unlocked.
This game mode I would say is at least 2, possibly 3 times as difficult as regular Exo Zombies. You have no HUD (no bullet count, no rounds, no orbital drop icon, only score.) They zombies are very much stronger. The only way to earn points is by getting kills, so points are very scarce along the way! If you go down in solo, that's it, no quick revive. If someone goes down in co-op, they just die. So take extreme caution!
So you start off on round 1 as always. the best way to earn points is to knife the zombies. It is two hit kill round 1 keep in mind. You want to make your way to get your exo suit as soon as possible. After you have your exo suit, make your way to the weapon generator. You are going to want something that can get you kills. (Lynx, RW1, S12, Blunderbuss etc.) Something that doesn't take alot of time to kill. Basically, you are just trying to stay alive long enough to buy perks (Exo Health, Reload, and Stockpile are the only three I would buy) and upgrade guns. The "safest" area to survive in would have to be the Spa. Run a train in spa and you should be able to survive.
Round 13 with Oz is going to be your first major challenge. You are definitely going to need exo health by this point. It is best to run a circle around the outside edges of this area because nothing that can kill you will spawn on this circle, besides zombies of course. Only go to shoot Oz when you know you won't go down, it's a risky move. When you manage to beat Oz, grab as many credits as you possible can, this is going to be very helpful to your survival!
You will now spawn back in the original map, at this point, the main goal you need to be looking for is to get the Blunderbuss, Nano Swarm, and Contact Grenades. Once again you will be safest in the Spa.
Fast forward to round 20, you have everything you need to kill Oz and you are ready to go. The Oz battle is tough, very tough. You have to continue to turn on all the generators after one use of the pad. He goes down fairly quickly if you shoot the ground over the pad, after activating it, with a nano swarm. Then just try and blow him away with the Blunderbuss. Max ammo drops do spawn, you just don't have a HUD so keep an open eye for them! The easiest way is to run in the circle like you did for the first encounter with Oz. It will take you multiple attempts to take him down to 3/4 health, 1/2 health, and 1/4 health. Once you kill him a DNA bomb will go off and you will be teleported back to the original map.
Boom, Achievement Unlocked!