Learn Finnish: Sinä luistelisit varmasti rohkeimmin. - You would certainly skate the most bravely.

The base adjective is rohkea (brave), and its regular adverb form is rohkeasti (bravely). To make it superlative (meaning "most bravely"), you drop the basic adverb ending and add the superlative adverb marker -immin, resulting in rohkeimmin.

Why is the verb form luistelisit instead of luistelet?

The -isi- in the middle is the conditional marker, meaning "would." To form it from the type 3 verb luistella (to skate), you take the stem luistele-, add -isi- (the final 'e' of the stem disappears), and add the "you" ending -t to make luistelisit (you would skate).

Are both varmasti and rohkeimmin adverbs in this sentence?

Yes, they are. varmasti (certainly) is a basic adverb formed by adding -sti to the adjective varma (certain). rohkeimmin is also an adverb, but it is in the superlative degree to describe the highest level of how the action is performed.

I sometimes mix up words for "surely" and "safely". Could varmasti mean "safely" here?