Learn Finnish: Hirviön isku on aivan arvaamaton. - The monster's attack is completely unpredictable.

It comes from the verb arvata (to guess). To form the negative participle, you take the strong vowel stem—which you can find by looking at the third-person singular arvaa (he/she guesses)—and add the suffix -maton. Literally, it means "unguessable", which translates smoothly to "unpredictable".

Why is aivan used for "completely" here instead of kokonaan?

Aivan is used as an intensifier for adjectives and adverbs, much like "absolutely" or "quite" in English. Kokonaan means "entirely" or "wholly" in a physical or complete sense, and it usually modifies verbs (like eating a cake completely). Since we are intensifying the adjective arvaamaton, aivan is the natural choice.

Is the noun isku (attack/blow) related to a verb?

Yes, it is the noun form of the verb iskeä (to strike, to hit). Many Finnish nouns are formed directly from verb stems by taking the basic stem and altering the ending, often ending in -u or -y.

Why is arvaamaton in the nominative case here and not the partitive?