My threat philosophy in GURPS Dungeon Fantasy is that some things can't be stopped or resisted.
In game terms, some threats:
- ignore DR
- ignore defenses that should otherwise seem like they'd work
- carry a followup that happens even if you do defend
- ignore defenses entirely
- have no resistance roll of any kind
This is usually done by fiat, although mechanically it would be written up as some form of Cosmic, a rule that allow you to bypass such defenses. Some are just standard spells, like Curse, which doesn't come with a resistance roll.
It also means that some threats:
- reduce DR
- reduce defenses severely
- have steep resistance penalties
These are usually more direct - armor divisors, defense penalties, high-level Afflictions, etc.
I basically do this because I think it fits the power level of Dungeon Fantasy.
When you run a Dungeon Fantasy character, you get 250 points, 50 points in disadvantages, and up to 5 points in quirks. That's 305 positive points worth of ability. In other words, you are a powerful individual. You are well-equipped to stamp all over mundane threats. Especially with threats that aren't up to your standards.
You are amongst the small percent of adventurers capable of even facing such threats. No-resistance attacks, ridiculously skilled foes, enemies that regenerate HP faster than a squad of armed men could easily whittle them off, etc. - you're one of the few who can even contemplate victory against some threats.
Victory isn't assured, nor is your chance for victory even guaranteed to be a fair chance.
If it was, you wouldn't really need 250 points. Higher point characters would be, even more so, unchallenged.
Plus, it's a balance mechanism for entertainment. If all Will rolls are unmodified (and you always get one), if all skill rolls are against your best skill in the best circumstances, if you always get your choice of Dodge, Block, or Parry . . . you really aren't under a real threat. It's just not going to be as fun.
So if you're wondering about those mean, unfair, cannot be blocked or parried and does damage that ignores DR attacks, or poisons that just do damage or reduce stats, or monsters totally immune to everything you normally bring to the table . . . it's all in good fun. It's what makes your victories matter, and makes the points you received to build your character matter.