Last time I played it they were still spawing after destroying the portals, but maybe less? Those things are made out of high grade explodium, so be careful. I had a Thunderhorse by the time I got that mission, so I had a lot of gals, and it still took about 1 1/2 turns to kill a pyramid thingy (they weren't all in one place, of course.
I am not sure if melee works but I had some of my gals working on them with their swords.