Learn Finnish: Pomon haarniska on rikkomaton. - The boss's armor is unbreakable.

To form the negative participle, you take the strong vowel stem of the verb. For rikkoa (to break), the strong stem is rikko-. Then you just add the suffix -maton to get rikkomaton.

Wait, doesn't the negative participle usually mean something hasn't happened? Why does it mean "unbreakable" instead of "unbroken" here?

You are exactly right! Words ending in -maton or -mätön can mean either that something has not been done, or that it cannot be done. Context tells you which one fits best. While rikkomaton could mean "unbroken", when talking about a boss's armor, "unbreakable" is the most logical meaning.

Why isn't there consonant gradation in rikkomaton? Shouldn't the double 'k' become a single 'k'?

The negative participle suffix -maton always attaches to the strong grade of the verb stem. Unlike the present tense where the stem weakens (like minä rikon, "I break"), this participle keeps the strong double 'k' from the basic form rikkoa.

Is haarniska used for modern body armor too, like a bulletproof vest?

No, haarniska specifically refers to historical suits of armor, like those worn by knights, or in this case, a fantasy or video game boss (pomo). For a modern bulletproof vest, you would use luotiliivi.