So, if you're referring to the Disney adaptation of the story, I've got a bit of bad news. You're off on the wrong foot from the start. While the film does a commendable job of bringing the ballad of Hua Mulan into some sort of context for people more unfamiliar with the story, there's a lot that doesn't exactly help to make things clear.

As for the story itself, there's basically two periods that are in the running for when the story is supposed to take place (based on mentioning of the Northern Wei): Either during the end of the Sui Dynasty/beginning of the Tang Dynasty (in the early 600s) or as early as the late 300s in the Northern Wei dynasty. This is bad news for the first scenes of the movie, seeing as the version of the Great Wall they present isn't something that was around until the 1400s (though, ironically enough, it would have existed in something like that form when the popular play versions were out in the 1600s). But as to the villains in the story, based on the region and time, it's most likely Mongolic groups in the north in a local war (with Mulan taking the place of her father in the khan's army that she lived under) or if we go with the end of the Sui, beginning of Tang, it's under an army allied with the Sui-Tang imperial armies, again fighting any of the local groups in that area (possibly Khitan, Jurchen, take your pick). The movie though, seems to conflate the story of Mulan and some earlier stories of "fighting off the invader tribes" that were popular throughout Chinese history as well. Many of those focus on things like the Han fighting the Xiongnu for instance, and therein lies the connection to the Huns. It's been a theory tossed around quite a bit that the Xiongnu and the Huns are one and the same group, but there's still nothing concrete to actually cement the connection (much of the focus in this particular area is aimed at looking at a possible Sogdian record connection. I believe "Sogdian Traders: A History" by Étienne de la Vaissière is one of the more prominent recentish books to make such a claim and connection).

So to summarize: Huns in the movie are conflated with earlier stories about the Xiongnu. Actual Mulan baddies are more likely Mongolic groups, maybe Turkic groups...maybe rebels. It's fairly unclear.