[tbone]

Most lamentable for the spider! But fate is against the creature. The scenario is written with the assumption that the PCs will win. The text suggests at least one huge spider per delver, as many as three. And that's just for the opening fight, a mere warm-up.

In a more "fair" fight, the spider should ambush from above, likely inflicting -2 to the prey's defense. It should try to push the PC into the webs (which could be a real fight-ender). The spider can also grapple (at +2 if the GM allows it to use 4 "arms"); that'll mess up the PC's attacks and defenses. Grapple the arm, and the PC's fancy sword is fully neutralized.

I'd avoid the "quick contest of Attacker's attack vs defenders defense"; that mucks with combat overall, essentially giving every attack a free built-in Feint. There's already a proper mechanism to handle the effect: Deceptive Attack. If the spider has TH of 16, it can drop TH to 12 or 10, and give the PC a -2 or -3 on every defense. The spider can also use AoA (Determined), and convert its +4 TH bonus to another -2 on the PC's defense. (A risky move, but it's a do-or-die situation for the spider.)

Lots the spider can do - but in the end, it's meant to be a challenge that DFRPG PCs can beat. It's destined to (probably) lose. The best answer is as you suggest: moar spiders.

It'd be fun to hear how the spiders fare in your re-match!