Step 1: Trim the Overhead
The Fate SRD has rules for Mobs and Teamwork starting here.
Everybody builds and runs their own Fate game how they like it; I for one favor a more diminishing-returns approach to ganging up. But anything that's in the core, accelerated, or toolkit SRDs are at least useful for teaching, reproof, correction, or training in GM righteousness.
(By which I mean they're a good place to start hacking your own rules from.)
So in this case a GM probably shouldn't be running 30 Average (+1) goons, but rather 5 Good (+3) gangs of six goons each.
Step 2: What's Going On Here?
Well, Molly is doing a thing that's going to take some time and everybody else is fighting.
So "Other Goals in a Conflict" (DFA p.190) seems like it's going to be useful here. Everybody else does the conflict thing, probably fighting more defensively and creating barricades and distractions, trying to run out the clock and keep them busy, while Molly engages in a contest to draw the circle.
Step 3: That's Numberbang!
So this is how I'd run it.
Per Molly's stunts, she can just veil herself invisible no problem, so she can start drawing that thing from word go. But she's in a contest, right? What's opposition? Unless she popped out of sight in blatant view of everything, nobody has reason to suspect, so I'll set a Great(+4) passive opposition to avoid drawing any attention to herself as she draws the circle, and decide on a total number of victories - three's standard, but this sounds like kind of a big deal, so maybe five instead.
Why +4? Setting Opposition (DFA p.209) recommends it as opposition for those cases where characters are doing dramatic things in unique circumstances and pulling out all the stops. If deliberately cutting yourself off from your allies to try and end a giant fight in one blow isn't an all-the-stops thing I'll eat several hats.
Also Molly's probably rolling at least +4 anyway, so this is going to be challenging without being impossible. And if she doesn't make it, I can be a dastard and offer her the choice of notching a victory anyway at a cost, that being that they've noticed you~~~ (And then one or more of the gangs of goons are going to be trying to sniff her out, either using their turn in the conflict to try and pierce the veil or passing their turn and acting as active opposition during the contest.)
And then Harry and other interested parties can fight more defensively, taking more Create an Advantage actions to set up defensive aspects rather than just trying to attack the goons, though of course I can be a dastard there, too, and offer anybody with some combative-sounding aspects a Fate Point to stop hunkering down and come out swinging.
And after Molly notches her last victory in the contest, she did what she set out to do and the circle's complete. (If she pushed herself out of hiding to finish the thing off, I'd probably run one more round of combat before it kicked in, for dramatic purposes.)