Corrosives have limits, they can’t just keep dissolving stuff forever.

Thus, the explosive assist for initial penetration. The type would depend on the composition of the concrete, you'd probably be more successful targeting the tension strength of the fibers or metals instead of the compression strength of the cement.

And what would “total failure” look like? It’s a mountain, it’s not going to just collapse into goo.

You don't need goo, you just need enough weakening that it no longer supports the 250' of loose rubble atop it and collapses into the interior space.

Coreium.

We drop an overloading nuclear reactor on top of the complex, and let the radioactive core China-Syndrome itself straight through, rendering the entire area uninhabitable for thousands of years.

Blood of xenomorphs from the Alien movies

My first thought is actually getting the corrosive substance onto enough of the concrete would be difficult

Yeah, if the concrete is 40' thick and they're only getting 10' of penetration with the explosives, then this isn't going to do much. But if it's 20' thick and they got through the first 12 with HE, the remaining 8 are going to have a lot of cracks to admit slow liquid death.

I have zero information on what the reinforcers are in the concrete, so shot in the dark is about right. Glass might be tough - unless you could deliver hydrofluoric acid effectively. Metals - we're not going to want to wait for iron to oxidize, looks like hydrogen embrittlement with HF again - so maybe that's the magic sauce. Nasty stuff, but that's what weapons manufacturers are good at handling and packaging: nasty stuff.

2000 lbs of HF poured on the surface isn't going to do much to the buried chamber, but 2000 lbs of HF delivered into the freshly stressed and heavily cracked concrete layer under all the dirt - that could be a problem for future use of the facility.