The ballroom is possible :D
Check out the MGS inspired stealth in UC3:
Syria below =
http://www.youtube.com/watch?v=umVk1Qlf7AE
Airstrip here =
http://www.youtube.com/watch?v=pQfay7Ng0s8
Seriously every encounter you are presented to stealth -> you can stealth.
In Airstrip when there were two guys standing. I threw a propantank and shot it in-middle of them. That worked. When I saw what this guy did; I had to try the ballroom again and yes. I was able to take out the Piano guy without being caught. I do tell you this though: It is bloody fucking hardcore to do it. I can't believe what a challenge it is XD and yes theres a lot of trial-and-error but when the game has such an amazing presentation, its hard to lose temper FOR ME personally. I've completed it 15 times and I just enjoy playing it xP.
