Bearing in mind that stealth, social engineering, changing the battlefield to somewhere else, or virtually any option that does not involve assaulting the pyramid is infinitely preferable from a tactical sense, and certainly from a long term consequence sense, if you HAD to go the assault route - and again, as many others have mentioned, this is a very not good idea - here is the dynamic:
-There are a ton of guards of all varieties, shooty, decky, magic, etc. there. Your only advantage in a fight with them is that they are spread out all over the building, and probably aren't just sitting around waiting for today to turn into a bloodbath, and will need some time to react. There is probably a small force ready for a quick reaction, but not everybody is going to instantly glom to where ever the runners are. Eventually they will, but that requires coordination and time. As such...
-Once the shooting starts, the runners need to be fast. No getting pinned down, no taking two minutes to pick a lock, no "oops, guess we'll have to run up ten flights of stairs because we got the wrong floor", no "thirty seconds to discuss this ethical dilemma" - nothing but steady movement punctuated by gunfire, grenades, breaching, and spells. And if anything stops them, they die. Lockdowns happen, guards converge, and it becomes an unwinnable battle. At that point you might offer them a surrender option...and if they don't take it...well, everyone always has a few good char ideas in their head, right?
-Chaos will have to be their friend. When this kicks off, neither the average guard nor the command center types know what is going on. If the runners start bee-lining forward, speed will help keep them inside the security's OODA loop, but their intentions and the nature of the crisis will quickly become identifiable to someone. At that point, big A has all the advantages because the runners are no longer inside the decision cycle, and vast resources are now being applied against an identified single problem.
The runners need to overload who is coordinating security's ability to think and comprehend the situation, and make it seem like the world is coming apart with no direction to the average guard. When this starts, there are going to be some guys in their bunks watching trid porn, and their first reaction is going to be "huh? Whats going on? I should find out what's happening or maybe shelter in place here until its clear." There are guards at checkpoints and stuff who, in the absence of outside direction, will just decide to hold their part of the building until someone clears this up. The runners need to keep those guys confused, cut off, and unmotivated to go looking for trouble as long as possible. At the same time, central security needs to be receiving garbled, contradictory reports about multiple problems, painted an unreliable picture, and even once they get the picture straight, it shouldn't be clear where to focus their efforts. You want to task overload their decision making capability - which includes fooling the analytic AIs. How to do that is up to the runners, but it will assist in preventing units from coalescing and central from massing against the runners. It may even have the first reaction teams head the wrong direction to go deal with one of the several other problems.
-A different way out. This should be obvious, but presumably as the lockdowns go into effect, the way the runners came in is no longer going to be viable..
-Legwork. Tons and tons of legwork. Because every step has to be near perfect to overcome the defense even before everything begins to track the runners, because you need to confuse the hell out of the guys with all the cards, and because you have no time to make or try a plan B before the wrath of big A exterminates you, the intelligence picture needs to be near perfect.
Do all that, and maybe, just maybe, they have a chance tactically. Of course, long term they're still dead unless they've got the a helluva bail out plan. There's no way they aren't being ID'd, and big A is not going to let them ride off into the sunset after the run is done.