Hmm.
Step 1: I toss the lightsaber into the cave.
Step 2: Yell, "Hello, Bride of Nine Spiders, what are you doing here today? Oh, trying to make it Ten Spiders? Congratulations!" at the top of my lungs.
Step 3: Wait for Shelob to decapitate herself with the lightsaber because even she probably has limits on how much she can enjoy spiders and paralyzing terror.