If I understand correctly, on a critical hit with its claw, the cat rolls 2d2-8 then applies the "minimum 1" rule, meaning he still only deals 1 point of damage. For the bite attack, its 2d3-8. So, critical hits don't help the cat at all.
If the commoner can take the cloak of the obyrith feat (Fiendish Codex I, p85), he gains damage reduction 1/lawful. This makes him cat-proof.
If the commoner can take the deformity (tongue) feat (Heroes of Horror, p121), he gains blindsense out to 30 feet. Hide and move silently automatically fail against blindsense.
Deformity (tongue) requires willing deformity (Heroes of Horror, p125), so fitting that and cloak of obyrith on the same commoner requires 1 more feat than a 1st-level human gets. Fortunately, swearing service to an elder evil gives a bonus vile feat (Elder Evils, p10), and both deformity (tongue) and willing deformity are vile feats.
Sure, you have to have the taint of a demonic ancestry in your blood, grotesquely mutilate yourself, and pledge yourself to the destruction of all that lives, but that's a small price to pay to run off a stray cat. And hey, once Atropus unmakes the world, there won't be any cats.