Having completed Zero Hour (both on Normal and Legend difficulties) when it was previously available and having cleared Normal difficulty Solo earlier today, I decided to give Legend a couple of attempts Solo as well (also allowing me to completed the Catalyst Switch Quest step ahead of a future grouped run) and on my final attempt was able to reach to just before the Super-Heated floor puzzle at the entrance to the Vault before exceeding the twenty minute limit. Knocking off a minute or two from each of the previous combat encounters as well as a minute from the TR3-VR section due to having more people flipping switches, I imagine a fireteam of three who are familiar with the activity and can traverse the jumping puzzles without too many deaths should be able to reach the boss encounter with a solid five to seven minutes remaining of the twenty minute limit (before factoring in Sword Skating and/or Strand). This may just end up being a situation where players who are having difficulty will need to reconsider their builds and put in the practice necessary to avoid needless deaths with the traversal sections, as frustrating as that may be for those with limited time to play in the first place. (΄◉◞౪◟◉｀)