Subjectively Invulnerable

GURPS 3eR had invulnerability, which was handy to make something, well, invulnerable.

It was:

You are completely resistant to damage from some attack form(s).  Knockback can still occur, however.  The cost varies according to how common the attack for is (see Absorbtion, CI p. 49, for examples of attack rarity):

Any kinetic damage: 300 points.
Common: 150 points.
Occasional: 100 points.
Rare: 75 points
Very Rare: 50 points.

Note that this a powerful advantage and should not be used unless ordinary Damage Resistance truly would not suffice.  Also note that this is an expensive advantage and that many attack forms have cheaper specific defenses (eg., Immunity to Poison vs. Poison, Doesn't Breathe vs. gases, etc.).

Special enhancement: The attack does not cause knockback. +20%

Expensive in this context is also 1/3 cheaper than 4e for the normal starting character too.

4e GURPS Update says:

Invulnerability

This advantage no longer exists. Buy a very high Damage Resistance and/or specialized Immunities instead.

 This came up because I just restarted doing the Fantasy Bestiary conversion and there's an Arab critter called a Siwanis.  It's like a musical cape buffalo without all the gregariousness the cape buffalo is known for...  The way you kill it is to target its horns.  They're well armored and hard to hit, but a mere 15 hits kills the beast.  Hitting it anywhere else is pointless.  4e tells me to use ordinary Damage Resistance to make this thing invulnerable...  But how far do I take that?

I hate this situation where, if I specify a DR, I have just made a "use more dakka" situation.

Do I scale it for a fantasy character?  Because a fireball can be pretty damn big in 4e.

Should it stop a tank round?  I like mixing genres, so making it immune to 120mm APFSDSDU would be needed.  Though a HEAT round might just blow them horns right off... 

One reason I set out to do this converting was to learn the differences in the rules and to make sure I could make every monster with a defined rule as written effect.

Right now I have it as DR 7,000 (Tough Skin).  That should keep the typical fantasy group from killing it.

I need to figure out the special rule that keeps the 15 hits to dead from being treated like 15 HP and all the effects on movement and consciousness.  I found it once, but...