Don't ask me. I came up with a whole game add-on just to stop stuff like that. In my experience the BBEG either can usually insta-squash 1 pc if played right. Or is doomed to die. It's only when you play the BBEG and try to "Spread out the damage" that he is usually not taken seriously. But If I was a king bugbear and 4 adventures ran into my throne room, I'm killing the mage and the healer first! Fight to the death! (because PC's will never allow villains to run away - they just want them to stand and die) - SO I would protect my healer, and kill their healer and mage! Nevermind the fighters!
But we never play like this. I used to attempt as a GM to "scare" the PCS and "play out the terror" by insinuation he COULD take somebody out.
That usually winds the BBEG up with his butt in a sling in less than 3 rounds.
70% of the time, happens everytime.
An because of this the same players will rush a dragon, without bows or the ability to fly themselves, and I would feel terrible If I had the dragon simply take flight, and breathe on them to death.
otherwise - I get to hear bragging about killing a dragon who fought the pc's with the worst choices (table friendly) ever.
Take another look at your BBEG. and stop trying to hurt/ stun all the pcs - And MURDER the heck out of one. Things at the table will change.
Or, you could use Epic Hero and Villain Feat Cards (shameless plug, but is it? because I put no link, ha!).
