I forgot to mention that the big bad will have a magical scrying link on the scene (and has ways of contacting the mercenaries). The big bad can also use magic on the scene, although would be wary about doing so if it means the Sorcerer figures out who the big bad is, since an attack would be a flat out violation of the Accords.
What kind of a fight can your PCs put up? Refresh isn't a very useful metric; it's possible to make 20-refresh characters who would get splattered by two gangbangers with AKs, and it's possible to make 8-refresh characters who can easily manhandle an armored column. If you have a WCV who can pump out a ridiculous amount of AoE shifts of Incite Emotion, and the opposition doesn't have a counter for that, it can very well shut down mortal soldiers however well-armed they happen to be. If one PC has Superb Resources and Windfall, and the merc leader has the aspect Anything For A Buck, it's going to be a real short negotiation.
OK. So there are 7 PCs, but it's rare to have everyone show up.
One is a Sorcerer. She has full evocation with the elements of spirit, water, and metal, as well as the Sight (her Rituals is just for exorcisms, and isn't going to be relevant in the fight). She's known for doing Harry style force shield blocks with spirit, and attacking with the metal element. But the player is good at being creative on her feet with magic.
Another is the Emissary of the Collective Unconsciousness of Humanity (which is a freeholding lord). He is very good at interrogating and whatnot, but in a fight he tends to just buff others psychically. He also has powers that lets him absorb the consequences of allies (and has Supernatural Recovery with the catch of it only working on consequences gained with that power). He has another power which allows all the PCs to act as if they have an upgraded version of Pack Instincts, which allows them to use Pack Communion if they are in the same scene (not just same zone), and they all use the highest initiative of anyone in the group +1.
There's a Knight of the Cross. She's got a really good Weapons skill, but her guns and athletics is less super amazing. I expect that she won't be as useful in this fight as she's been in ones against supernatural baddies.
There's the "Retired ex-biker" (his high concept). Pure mortal, with 9 free refresh. His athletics is 5, and his guns is 4. His player is a very strong tactical thinker, and a lot of his aspects are handy in a gun fight.
There's the half nixie changeling. Her main response to a fight is to turn invisible, sneak around, and then swing at someone with a weapon and her inhuman strength.
There's another pure mortal, who's an RCMP constable. He's got reasonably good athletics and guns, but not as good as the ex-biker. If he gets a chance to call for backup though things will get bad for the mercenaries.
The last is a mortal with faith powers. He's got Fists at Superb. The mercs won't be suspecting him since the big bad doesn't know about him, but at the same time I don't see him making a huge impact on the fight.
The PCs know how manoeuvres and blocks and whatnot work, and are good at using them. They may or may not be expecting a fight, depending on how various rolls beforehand go.