I've been playing alot, almost all mystery heroes. I'd like to say I'm improving but I don't really see a way to get win to loss ratio.
With nobody teaming up, the best way to win is to just suicide on a priority target and try to take them with you. Any character that auto aims is priority 1 (sym, torb, moi), with healers next. Orisa is a fairly high priority target as well. The rest take actual skill to play so aren't any threat really aside from the occasional lucky shot. Well maybe junkrat.
Win/Loss aside, Mystery is really about practice and learning heroes and fun. It really would be 10 times more fun if they would remove heroes that need a hard counter. Get a torb on the payload with any kind of shield and it's all over 9 times out of 10. People don't live long enough to build up ults.
That and take out hanamura / anubis / moon. I leave those maps when they come up, but I'll bet if you look at the stats 90% + of games defense wins. You have to have the right hero combination to take point 2, and have to hope the defending team has something terrible. People used to just suicide until they got a reaper or something decent, but you can't do that anymore.
BTW, I caught up on all the OWL games. The houston vs london game was awesome. All that counterpunch and hero switching was great, and lots of badass brawls and big plays.
I could almost skip the first game of the matches. Hanamura is just a terrible map and needs to be redone or ditched. All the assault maps are bad, it's just a bad mode, but at least Anubis has some good tactical attacks from the teams. The early stage 1 dallas vs seoul match had some awesome back and forth on Anubis.
It would hurt seoul to ditch assault. They are the kings of the cheesy stall tactics. Which are of course not fun at all to watch.