Update: An easier method to obtain Übergold on Bathhouse has been found. I have updated the video below with the easier method.
I figured since I unlocked this challenge I would try and give some tips that I used to unlock it.
Getting headshots and keeping your combo going is key to unlocking this challenge. There are many maps where if you don't get good combos in them, you won't have enough points to get Übergold,
With the exception of the beginning of the Prison Docks challenge, you want to always alert the commanders, and kill them last. Without getting alerted, there won't be enough enemies in the level to achieve Übergold.
Become familiar with where the armor and health packs are located. On Über difficulty, your health drains very quickly, so it is best for whenever you have low health to quickly find the nearest health or armor.
If you think you have killed everyone and don't have enough points to get Übergold, check all around the map to make sure no enemies are hiding or are stuck somewhere. You should also wait a little bit to make sure all enemies have spawned.
Lastly, have fun, this challenge will be very annoying if you don't enjoy the challenges. Depending on your skill level and how the AI decide to act, it can take a while until you can pull off the combos you need to get the Übergold.
I'm including my runs that I achieved Übergold on, so that they can hopefully guide you on good strategies for specific maps.
*If you vote negative, please leave a comment or message me why so I can hopefully change the guide to make it more helpful.
Research Center: 13,000
Prison Docks: 30,900
Keep Foyer: 27,300
Cable Car Platform: 29,800
Paderborn Bridge: 24,900
Welfburg Square: 19,000