To get the 'good ending' in a scenario, you need to choose the right dialogue to use. For Mystery Doors of the Magical Land 2, the first time you have to select dialogue say "You, too?" and the second time say "Flee with me!" Then, assuming you do what the mission actually says ("Knock out SFX Prop W2 with Astonish. Don't knock it out with any other move") you should get the good ending.
As for how to complete this mission, I like using Toxic, and then spamming Astonish, and hoping it doesn't Rest. I think if you kill it with Toxic damage (if you used Astonish in that turn) it will still count as killing it with Astonish, but I could be wrong so don't quote me on that.
Source: Bulbapedia has some stuff about it