Learn Croatian: Junak je hrabro napao neprijatelja i ubio zmaja. - The hero bravely attacked the enemy and killed the dragon.

This sentence describes a sequence of completed past actions: first he attacked, then he killed the dragon. To show that one action finished before the next one started, Croatian uses perfective verbs. If we used imperfective verbs, it would mean the attacking and killing were happening at the same time or repeatedly.

Why is there only one je in this sentence even though there are two verbs in the past tense?

You don't need to repeat the auxiliary verb (je) when connecting two past tense actions with i (and). The single je applies to both napao (attacked) and ubio (killed).

Both neprijatelja (enemy) and zmaja (dragon) end in -a. Since they are the direct objects being attacked and killed, shouldn't they be in the accusative case?

They are in the accusative case! Because the words for enemy and dragon are living masculine nouns (animate), their accusative form is identical to their genitive form, which ends in -a. If they were non-living masculine objects like a sword (mač), the accusative would just look like the nominative.

The verb napao (attacked) ends in -o. Where did the usual -l for masculine past participles go?