Which I didn't have. Yeah, I set up the terrain, with the ledge and everything, in a way that gave them a chance at dealing with it without getting completely hosed, but (to be entirely honest) that wasn't what I was thinking when I set it up like that. I was thinking "hey, Ax has that neat gliding ability, and his player seems really into it, I should give him an opportunity to glide into combat." So I did. Only once the scene had gotten rolling did I realize how smart of me it was to do that.
I didn't have a plan. I never have a plan. I didn't know whether they'd sneak around it, or figure out a way to kill it, or just charge in and get themselves all killed. I hadn't even given much thought to how they'd deal with it. I knew it was possible for them to deal with it somehow, even if that might involve bringing in their parents and mentors, but I figured it was up to them to work out exactly how that was going to go down.
It's not just that I know the players will mess up any plan I come up with, though that is why I originally adopted the policy. More fundamentally, any plan I come up with can't possibly be as interesting as one designed by four or five people very motivated people. If I come up with a plan ahead of time, I'd be tempted to say that whatever crazy idea the players came up with was "wrong," and that would make the game less interesting. It requires a certain comfort with improvisation, but I developed that quite quickly when I realized it would get me out of doing work.
So I don't plan. I come up with ways to motivate my players, and then I get out of their way. Less work. More fun. Your mileage may vary, as always, but it works for me.