BBEG Fortress

So the BBEG is effectively a computer. She can't do much personally, but she's at the bottom of a pit surrounded by mecha-mooks. Like, a couple hundred guys who're about CR 4, against a 3-person level 11 party. They don't have the resources to just blast their way through this, and I need a way for them to be able to win, or at least do meaningful damage.

Thoughts so far:

Generator. She's a computer, and all her stuff runs on electricity. Find generator, break same.

Communications. Her mooks are a lot like undead. They're mindless apart from her orders, so if the party could tear down whatever radio she's using.

What have you guys done with BBEG lairs to make it something other than an endurance test?