Learn Italian: Ripetiamo ogni parola difficile cento volte! - We repeat every difficult word a hundred times!

Because ogni translates to "every" or "each", and in Italian it must always be followed by a singular noun without an article. If you wanted to use the plural to express the same idea, you would have to switch to tutte le (all the): tutte le parole difficili.

Why doesn't the adjective difficile end in -a to match the feminine noun parola?

Adjectives that end in -e in their base form (like difficile, forte, intelligente) are used for both masculine and feminine singular nouns. You don't change them to match gender. They only change to -i when the noun becomes plural.

Could ripetiamo also mean "Let's repeat"?

Yes! In Italian, the noi (we) form of the present tense is also used to make suggestions, equivalent to "let's..." in English. Because of the exclamation mark at the end of this sentence, "Let's repeat every difficult word a hundred times!" is a very natural alternative translation.

Does the number cento ever change its ending, for example to match the feminine plural volte?