Well, aside from the fort save every time you take damage or die...
The rules that you allude to aren't actually in the CoC d20 rule book. Perhaps your Keeper was using house rules?
There
is a Massive Damage rule, but it isn't applicable
every time that a PC takes damage — only if they take more than 10 damage as the result of a single attack. That said, yes, this is the
one thing can undo a high level CoC d20 character.
There is
no Fortitude save for Dying in CoC d20. There
is a 1d10 chance to stabilize after being reduced to zero Hit Points, but this mechanic actually
helps the PCs survive, as opposed to hastening their demise. A successful Heal check automatically stabilizes them.
And, of course, none of that prevents a high level CoC d20 character from hitting almost every time in combat (or being almost impossible
to hit in combat).
Frex, a 15th level Defense Option character has Base Save Bonuses of +5, +9, and +9, with two attacks per round — the first with a Base Attack Bonus of +7 and the second with a Base Attack Bonus of +2, respectively.
A 15th level Offense Option charcter, OTOH, has Base Save Bonuses of +5, +5, and +9, with
three attacks per round — the first with a Base Attack Bonus of +12, the second with a Base Attack Bonus of +7, and the third with a base attack bonus of +2, respectively.
By level 15, Stephen Hawking would have 18 Core skill ranks and 7 feats — all of which I assume would be Skill Emphasis. This is the one feat in CoC d20 that boosts skills — and it only grants a +3 bonus to one skill for each time it is taken. It does
not stack and may not be purchased for a single skill multiple times. I'll be generous and assume an Intelligence rating of 18, which grants a +4 bonus to all knowledge skills.
This
still wouldn't be enough to cover the fields of Hawking's multiple degrees, both honorary and official, while modeling his insight into such things. And our character is
already into the deadly combatant range if he's an Offense Option character. If he's a Defense Option character, he's already
very hard to hit.
I
think this is how AB came up with the Stephen Hawking Badass Build at Level 20. There simply aren't any feats in the game that allow one to build up
lots of knowledge skill without also taking lots of levels (which, in turn, results in lots of combat bonuses).
As I said before, none of this is an issue at lower levels, but only the Massive Damage rule stands between high level CoC d20 characters and super-heroics (and, even that doesn't post
too much threat once you're boasting high Base Save Bonuses).