The H4 Black Hole quest can be solo'd as op, though can be tricky and all the time you save skipping trash you waste on the last champ boss.Think il only bother doing it once a week.
edit: and thanks for this post btw
Care to share your methods for soloing the mission? I beat the bodyguard to get the key (by using the same method I used on Raxxus), but in the final room I couldn't solo 3 strongs and an elite - I kited them around a while but they eventually wore me down. And I think there's another wave of strongs and normals after that before getting to the champion. Feel free to send me a private message if you want.
I like having the option to solo even if it's time-consuming for the times I can't find a group.