Thanks for the response. I have minor image, silent image, and ghost sound at my disposal. We engaged with the drake in the area, so I have some experience, and it is in the area so helps with the believability. But there are obviously limitations with making a creature vs environmental effects.
How well this works is, as you say, depends greatly on the how my GM will handle how the NPCs react. So I'm hoping that I can work out a couple of plans that look promising.
1) Fog/smoke which gives cover to the humans
3) Approaching reinforcement via ghost sound/image
4) Some ethereal creature that can be used for fear and chaos
Any other ideas?