Question: In my story, I want a creature to be immune to bullets, but giving it bulletproof skin seems too obvious. I want it so that you could shoot the creature a few times at its seemingly vital spots (the head, heart, etc.), but it won't die right away and can still escape or attack you. The best way to make sure it dies quickly is by draining most of its blood, which means you have to get close enough to cause a large wound with a bladed weapon.

Yes, I know you can just use a really big gun or explosives to achieve the same effect, but in this scenario, the options are limited to something relatively cheap, lightweight, and doesn't cause a lot of collateral damage. To give you some image, the creature is about as agile, as strong, and as big as a polar bear.

Preferably the reason it's immune to bullets is based on real animals, or at least something that could exist in the natural world.

Answer: 
Amorphism, if the creature has no vital organs but rather is composed of "full function" non-specialised cells that all do a bit of everything but require a certain critical mass to function as a whole. Then point damage like a bullet wound does little to no damage to the overall organism while cutting it up with a sword reduces individual sections below survival mass killing chunks of it off until the whole no longer has enough cells to grow back. Something like the [Oozes](https://www.d20pfsrd.com/bestiary/monster-listings/oozes) of Dungeons and Dragons or Calvin in [Life](https://en.wikipedia.org/wiki/Life_(2017_film)). Real world examples of amorphic life are the [Sponges](https://en.wikipedia.org/wiki/Sponge) that have mutable cell functions based on location rather than a fixed function for a given cell.
* **Resist piercing:** Projectile immunity is usually best achieved by avoiding getting pierced. Some ways to shed bullets include:
	+ hard surfaces
	+ angled surfaces
	+ low coefficients of friction
	+ active deflection (springy bristle like hairs could provide this, as would orienting body surfaces)
* **Resist momentum:** Projectiles impart momentum and that is the source of damage. Ways to resist imparted momentum are:
	+ having a much larger mass
	+ "low viscosity" or low yield strength (if we fail at a point then we can't spread the momentum to areas adjacent to the point as easily)
	+ mass shedding. Having a skin layer absorb momentum and shear off means we have ablative armor that carries momentum away .
* **Die to bleeding:**
	+ Blades would have trouble finding purchase on a surface smooth enough to help deflect bullets. But conversely that may make catching on the non-slick wound easier. Repetitive damage in a single area is generally not good.
	+ In a creature with low viscosity overall it would lose internal fluids quickly. To not die from a bullet it would need something like a self-healing skin, porous and highly elastic. So bullets could open a pinhole. But a blade would open a larger wound which would be fatal. Alternatively you could add something like platelets that congeal on contact with air. Larger wounds are just harder to congeal.
	+ Extremely hard materials could be brittle (typically a standard trade-off) so the higher impact from a blade or hammer could be enough to break it where a bullet would not. Death by bleeding then just being standard.
	+ Extremely large objects absorb the momentum outright and the bullets and blades can only go so deep. But Blades have the potential to hack away chunks of material and dig deeper. Something a bullet has a hard time doing.

Hardness and mass tend to hand-in-hand material wise. So you could go with a tank-like creature. Alternatively something akin to a slime with special skin would work as well. Mix and match if you need protected areas (skull for example). Other options exist as well.