This is stage-specific and can only be done on: Hypno Baron's Castle, stage 2 (the side-scrolling outdoors run).
Bonelegs is a spider/female hybrid that defends itself quite frequently. It may be annoying, but only with the Spider Venom artifact can you do any damage as a Spider. Keep a bit of distance between you and her and spam shots of three at a time. The first should be spaced by half a second for her to defend and then start gloating, while the next two should hit her. It'll only do 3 damage each shot, so you might be doing this for a good minute, but once she falls to your Spider Dance, the trophy will pop.