You have a small knife

You take your knife out of its sheath. You normally only used it for cooking, or other routine tasks, but it would do just as nicely here. Locating where the waisteband is tightest, on her thighs, a quick cut and a tug to manually rip it further does the trick. Without anything to hold them up the pants pool in a heap on top of her boots at the bottom of her shins.

She feels what just happened and freezes in surprise, giving you a moment to appreciate your work. She wore underwear of the same type of fabric and style as her bra, black, but also a black garterbelt suspending her stockings. She must be regretting that the garter couldn't have held up her pants too though.