“The perps would be taking fire from all points of the compass”
That sounds like a circular firing squad.
One extremely-vulnerable class of targets come to mind: high-school and college graduation ceremonies coming up in the next few weeks. Schools and colleges are gun-free zones, and thus are tempting targets for terrorists.
As far as "taking fire from all points of the compass", all they have to do is pick a spot where a large crowd is expected to be, and which has buildings overlooking the area, into which they would be able to infiltrate and set up a barricaded sniper's nest.
Alternate scenario: do you think it would be hard to smuggle in a mortar plus 20 rounds for it, plus an expert crew to man it? Visualize the effect of a crowded sports stadium taking mortar fire. More people would die from being trampled in the panic than from the mortars.