You've read 'The Shrinking Man'. Of course you have. You did all of the research before embarking on this experiment, even going so far as to look at fictional representations of your environment. And now, here you are, fighting a spider in a manner not dissimilar to that book.
Only you are on a level playing field with the beast here. Matheson's hero only triumphed with the aid of his environment, and this level plain offers no such opportunity. Trevorina, emboldened by the giant's departure, stabs forward with a flurry of limbs, forcing you to turn aside one, two then three talons before your left foot slips and you fall to your knees.
You've no choice left here. Unaided, you'll die here. But you do have one surprise literally up your sleeve.
First you toss your javelin at the spider, the surprise move at least forcing her to go back on the defensive, ducking to protect her eyes. Those moments will be what you need to deploy your ace in the hole.
You had hoped to save it for bigger game. Truth be told, you'd almost been tempted when the chipmunk attacked, and in the future you may well wish that you hadn't wasted it now. You'll be happy just living long enough to experience that regret.
It's relatively simple as far as firearms go, more spectacle than substance. The discharge and impact are a lot of flash and noise, more of a firework than shotgun, and you count yourself lucky you managed to devise one that works at this scale. Trevorina takes it full in the face.
Your original intent had been for it to function as a distraction for a larger predator, perhaps enough to even scare them off. To the relatively fragile form of Trevorina, it is rather more devastating, blasting half her eyes from her head and staggering her back in shock.
You charge forward, snarling with pent-up anger releasing, and are upon the spider before she can recover. Your bare hands dig into the empty holes of her eyes, tearing more and more of her fragile innards into the light. You punch at her functional eyes, and the flailing limbs that lash at your back barely scratch you - she's dying, losing her strength.
Trevorina, your great nemesis, dies in your embrace, her ichor flowing freely as you drag yourself from within her. This is a triumph. The exhileration, it is overwhelming! The whole experirnce of this shrinking trip has been worth it for that alone. You are INVINCIBLE! Like unto a GOD!
The world trembles beneath the incoming tread of the giantess, and you freeze as the shadow of a true god engulfs you.