I started out with my speedlight mounted on a lightstand, set at approx. 45 degrees behind him so as to give some controlled backlighting - and to hopefully help combat the wind that was buffeting him around. I set the speedlight manually to its 24mm zoom setting, the head tilt to 45 degrees and placed a diffuser dome over it to further soften and spread the light. This worked pretty good for about 6 photos, then I had to grab my flash and stand due to a sudden downpour of rain.
As the rain was now turning more persistent and heavy I switched to on-camera but keeping the flash settings the same. Now I had a water-logged spider and web to shoot, so I made the most of the opportunity, shooting off several photos.
Here's a photo taken with the flash backlighting the spider:
And here are two photos with the on-camera flash of a rather damp spider: