The way sts wants the players to solo is to make a party with a real player (not being yourself) and not join the same map as you you do..
I've asked numerous times on the shoutbox about a solo option and that's the awnser I got every time..
If they wanted this to be a normal function they would have told about it or implemented a real solo option.
My guess it that they will call this an exploit and remove the option to do this.

Just my 2 eurocents