This managed to be easier for me than the Reunion (easter egg) trophy. It really just comes down to playing the game different than regular zombies. I was able to get it first try on solo attempt. Not only that, I had MK 20 weapon before round 19 and had (key word) enough credits to open the rest of the map. It made all the difference. Here are tips to help you do the same:
1. Play it solo. I tried doing this 4-5 times in co-op. Furthest we got was round 19. Playing this with others with all the handicaps put on the characters is just too much to ask. Not only do you need to play at your best, but so do your partner(s). Also, the more players, the more zombies and that makes it harder. You can't revive and if you are constantly together, the odds of getting cornered and dying are greater. Statistically speaking, it's just too hard to do with more people. Go solo.
2. Melee kills should be priority up to first infection round (7-11). You get 130 points for melee kills (260 with 2x multiplier). Considering you only get points off zombies for melee kills, your goal should be melee kills. This is vital go get weapon to MK 20.
3. Roll for ONLY 1 special weapon off 3D printer. This is the only gun you will be using. Ideally you want the CEL-3 Cauterizer, it can 2 shot kills zombies or more preferably, 1 shot weaken zombies so you can 1 hit melee kill after for most rounds AND at MK 20, it kills Oz quickly and IMO, is better than having blunderbust. Do not bother working with a gun that is mediocre. It's either a power weapon or restart.
4. Limit your spending. The only perk of Double Feature is that upgrading weapons only costs 1,500 v 2,500. Other than that, you have to be very selective with your spending. I suggest only buying Exo Health and Exo Reload. That said, the doors you should open (in order), is Landing>Reception>Promenade (3D Printer)>Annex>Galleria>Tidal Generator than got back to landing and open Concierge>Great Hall **DO NOT OPEN Concierge from Reception**.
5. Utilize orbital drops and goliath. Preferably, you want sentinels to drop or a standing turret. These will be necessary to survive the first infection round without having to open the spa. You will have 2 goliath drops before round 20. Use the first 1 after the first Oz battle and the second should be used when the 2nd infection round starts. The great thing with the goliath is that you can easily get through 1 1/2 rounds before it self destructs.
6. Camp somewhere that has a (working) teleporter or tube is easily accessible. This is partially why opening Reception and Concierge from Landing is key v opening Reception/Concierge from Landing and then the other. You want to have access to use the tube because 1) it's free; and, 2) zombies can't attack you.
Game plan: Play on landing up to level 3. All kills must be melee. You can use grenades on round 3 as needed, but it's easier to train zombies and pick them off as you go. Open Reception>Promenade and use 3D printer on round 4 (do it as last resort on round 3 if needed). If you get a bad gun, restart. Next step is to open annex>galleria and get exo suit. Then open Tidal Generator and buy exo health, then exo reload. Orbital drops will occur between these steps, be sure to pick them up. By end of round 7, you should have exo health + reload. Go back to Landing, open Concierge>Great Hall and camp near the power generator. If things get too hectic, jump and go up tube to Landing then camp in Reception. Because the door is shut between Reception and Concierge, zombies have to come down through Landing, and you can easily pick them off this way. This is your bread and butter on getting through harder rounds. Ideally you will want to be doing this on the concierge side because it is less likely zombies will spawn behind you and even if they did, it's easier to position yourself so you can just turn around and look without having to move away. Be sure you are upgrading your gun as you go. When the infection round happens, camp in Great Hall and activate your orbital drop when more zombies start showing up. Try not to do it immediately at the beginning of the round because it can expire before end of round. You should get through the round with ease. Keep note of where the goliath drops (either landing or tidal generator), you will need to head over there after first Oz battle. In the First Oz battle, buy contact grenades. After Oz battle, use goliath for that round (be sure to melee as much as possible). After this, open the lounge and turn on the power and buy teleport grenades (only necessary before 2nd Oz battle), this will allow you to teleport from the great hall if you can't run to landing safely (and if you do, just run over and camp reception and pick off zombies as they fall down from landing). From here, it should be smooth sailing and all you need to do is defeat Oz. During the battle, try to save orbital equipment for last 25% health if possible but definitely use them as you need too.
Hopefully that helps.
Happy hunting :)