Banana slugs have no shell and cannot outrun anything. Their principal defence is mucus, secreted in two functionally different forms: a thin lubricating slime used for locomotion, and a thick, sticky secretion released when the animal is disturbed. The defensive form is highly viscous, clings to fur and mouthparts, and absorbs water to swell.
Animals that mouth a banana slug report, in the human case and in observed predator behaviour, a numbing effect on the tongue and lips. The combination of an anaesthetic sensation and slime that cannot easily be wiped away makes the slug a poor prospect for most vertebrate predators. Raccoons are reported to roll slugs in soil before eating them, which soaks up the mucus.
The locomotory mucus is separately interesting. It behaves as a shear-thinning material, flowing under the muscular waves of the foot and setting again between them, which is how a slug can climb a vertical trunk. The frequently repeated claim that the slime is a strong anaesthetic in a pharmacological sense overstates the evidence; the numbing is a real but poorly characterised sensory effect.