Have any of the characters gotten their hands on some "I really don't want to use this, but.." items, like a little glass sphere that can summon a Demon, if you are willing to "pay his price" afterward? The characters may have all the tools they need to change the situation from hopeless to "survivable, yet more complicated."
Here's how I'd see a situation like this turn from one "We're all gonna die" mode into another "Uh.. maybe not. Maybe." mode, with the GM playing fast and loose:
Monster lair. Major baddie, Jar-Snapper. Party has been fighting him a few rounds, Jar has big nasty friends, and he is demonstrating that he can probably win this one all by himself. Handily. It's clear that something drastic needs to be done.
A conscious PC pulls out (Rent-A-Demon) sphere, says to it, "All right, I need your help, come forth Krexus and fight my foes!"
The earth rumbles. (The GM gets a crazed look in his eye.) A previously less-noticed NPC, one of the uninvolved guards at a further-away tunnel entrance to the lair suddenly goes pale, and yelps,
"No. No! Crap!"
..and bolts away down an unexplored tunnel.
The big baddie is frozen, seeming suddenly shocked and deeply, deeply disturbed. A head suddenly erupts from the side of Jar-Snapper's neck.. a head which is larger than the baddie's. Much larger. The new head's sudden appearance arrests the attention of all the NPCs. (Combat ceases.)
The warty yet horrified face of Krexus, meanwhile, is frantically shaking itself from side to side in the sphere. He half-moans, "I'm not fighting *him*! You're on your own!"- and the sphere shuts down.
How do the conscious characters take advantage of the situation? And later: What exactly happened there?
--and that's just one way a GM can "save" the PCs. Unexpected consequences, holdout items, disturbing new revelations, and opportunities to bluff or run - good times.
But it does rely on the characters to realize when it might be a good time to use the item "they'd rather not." Give the GM something to work with, people. When you have the chance to obtain that new, experimental "Wand of Greater Wonders", do it. Don't sell the Enigma Device.. hang on to it a while. Yes, it sometimes cries at night. Keep it anyway.
And for the love of wonder, doesn't anyone in the party have a maxed out Bluff skill? Preparation needs to meet Opportunity wearing a freshly-pressed suit with a rose in the lapel sometimes, it helps quite a bit.