Ignoring Armor once and for all.

[Kromm]

From p. B47:

Hardened: Each level of Hardened reduces the armor divisor of an attack by one step. These steps are, in order: "ignores DR," 100, 10, 5, 3, 2, and 1 (no divisor). +20% per level.

To paraphrase Armor Divisor, p. B102:

Armor Divisor . . . Modifier
(2) . . . +50%
(3) . . . +100%
(5) . . . +150%
(10) . . . +200%

Note how the modifiers are priced at +50% per canonical step. The next steps would logically be (100) for +250% and "ignores DR" for +300%.

Finally, to precis Cosmic, p. B103:

Irresistible attack. Your attack does negate the target's protection; e.g., an Innate Attack that ignores DR. +300%.

That's simply an explicit statement of the missing final step of Armor Divisor.

When Powers says, "DR with Cosmic subtracts from 'irresistible' attacks with Cosmic," it means exactly what it says. DR 100 with the +50% version of Cosmic counts as DR 100 vs. an attack with the +300% version of Cosmic. Against an attack with a (10) divisor, that Cosmic DR acts as DR 10, because the defensive +50% version of Cosmic isn't Hardened, and does nothing againt other armor divisors. Somebody with DR 100 (Hardened 1, +20%) gets DR 1 vs. an attack with the +300% version of Cosmic, because he shifts "ignores DR" to (100), while he gets DR 20 vs. an attack with a (10) divisor, because he shifts (10) to (5).

If you want an attack that not only ignores DR but also ignores Hardened, then add another +50% to your +300%.