Learn Finnish: Tämä voittamaton loppupomo on minulle täysin käsittämätön. - This unbeatable final boss is completely incomprehensible to me.

You use the strong vowel stem of the verb and add the suffix -maton or -mätön depending on vowel harmony. For voittaa, the stem is voitta-. Adding the suffix gives us voittamaton, meaning unbeaten or unbeatable.

Does a negative participle like voittamaton have an active or a passive meaning?

It can actually be both. In Finnish, this participle can describe someone who does not do something, which is active, or something that cannot be done, which is passive. Here, voittamaton has a passive meaning of unbeatable. You rely on context to tell you which makes sense.

What is the root verb for käsittämätön and why does it use the -mätön ending?

It comes from the verb käsittää, which means to comprehend or to grasp. The strong stem is käsittä-. Because of vowel harmony rules, the front vowels in the root dictate that it must take the front-vowel version of the suffix, resulting in käsittämätön.

Why do we use the allative case in minulle here?

In Finnish, expressing how a situation or thing appears, feels, or is perceived by someone often requires the allative case (-lle). Just like the common phrase Se on minulle vaikeaa (It is difficult for me), you use minulle here to specify who the final boss is incomprehensible to.