If you just want it as an encounter, then either have it be a slightly more powerful creature or group that is at some disadvantage, or a less powerful individual or group that has some advantage.
For example, if it is more powerful, the players may have the advantage of knowing ahead of time what their opponent is, or its objective. If it is less powerful, then the players might have to approach where their opponents have sentries, or have the aspect "home ground" that they can tag.
You can also have a group whose objective is going to be primarily to get away, rather than fight it out.
For example:
Dangerous beastie has come out of the nevernever, where the Veil is thin, party comes across or is called to the site of one of the attacks, gets either a thaumaturgical link to track, or a scent trail a were- or changeling can track. Party can try to set up bait of some sort, or trail directly. Whoever can keep up with it might not be able to take it out, but perhaps they can kite it away from people, and to a kill zone.
A followup might be to try to find the hole it came through and seal it.
Think about what skill or aspect is drawing the players in.
Do the players have high Contacts skill? Maybe the police call them in for a consult. Or the occult bookseller reports some really creepy buyers. The clued in barista notices some customers with no auras, or distorted ones. So the initial encounter may involve just drinking some coffee, observing and trying not to be observed.
Do the players have thaumaturgy or tracking skills? Have them follow something from an incident using those abilities.
Or are the players brought their by Fate, pushed by some sponsor or Faith ability? Do the players actively patrol, looking for trolls, or Red Court?
Some of the challenging encounters could be with individuals or entities that could be allies. There could be something else hunting or tracking what the players are interested in. Some clued in individual or group that investigates bizarre murders that the police can't handle. Such a group could be an ally or obstacle.
There could be challenging encounters with groups or entities that could be co-belligerents. Not allies, because of their nature or agenda, but at least on some battlefields they would be fighting the same things. Maybe the White Court Vampire is staking out the museum because he knows a particular warlock is interested in a very nasty item that is there. Or maybe the warlock the party is hunting offers to help versus an imminent demon invasion sponsored by his former associates, if the party is willing to give him a head start to leave town afterward.
Some of the challenging encounters could be with people the group would nominally protect, but are kind of difficult.
