I think there's a difficult backroute that doesn't use the left side of the level. I can't see how to use the left side, it looks like it would need 1 more builder.
You certainly aren't bad, so evidently my levels are good.
I'm still somewhat surprised that you haven't passed any though.
I fixed a backroute in my pack. Same url as before. It would have been very hard to execute... but it would still count as a backroute.