And I only say this, cause I thought it was cool, and I basically got the idea from you, so I wanted to give credit where its due, hehe...
You could basically make an Aurora Borealis kinda bridge, and walk over it with all the pretty lights... maybe even invoke it a second time, to daze and confuse the bad guys as they stare on in dumbfoundedness...
But basically, using the earth element, instead of real rock ((which sounds like it may be a little scarce in the current example)) using the Earth's magnetic field, imbuing it with a lot of extra juice, then steadying it to be used as a literal bridge..
I could see it as one or maybe even two castings...
The two casting version, both would be earth evocations, meant to place aspects on the scene..
The first, staring at 3 shifts for the maneuver of "The Aurora" as it visibly gathers the earth's magnetic forces, then however many more you'd think you need for added time, though technically, you'd only need it to be tagged in the next spell mechanically.. I'd prefer to keep both spells running though, cause it would just make more sense thematically...
The second spell is where a lot more heavy lifting comes into play.. Another start at 3 shifts for the next maneuver, "Bridging the Gap". Then another 2 shifts of energy, to create an effect large enough to effect the whole zone. Finishing off with probably a base of at least 2, maybe even 4 or 5 for duration, so everyone can get away. So thats a base of between 7 to 10 shifts. Tag "The Aurora" to help you with a +2, I doubt any GM would argue that, then Invoke for Effect your Aspect of "Bridging the Gap" with your free tag for bringing it into play...
pg. 106, Tagging
"A tag is a special move that you may be able to
do when you’re invoking aspects other than your
own. Whenever you make a roll to gain access
to or create an aspect, as per the list on page 105,
you may invoke it one time, and one time only, for
free—as in, you don’t spend from your pool of
fate points to take advantage of the aspect."
It then goes on to say that a tag is only subject to the key limitation, that it must be used almost immediately after the aspect has been brought into play. So, from this, I would assume that you Could tag for the normal +2 bonus, or you could Invoke for Effect, or any other effects normally allowed through invoking... Anyway, sorry for the rambling... Would love to hear if my idea is completely impossible by the rules, and if it even just sounds flat out stupid, hehe >.<