Ah. Here we go again with the absolutes like "never" and "100% guarantee".
The unfortunate truth is, you can't 100% guarantee




, dude. Let me pull my own example:
I had a party of that level take on an adult white dragon in its lair w/ lair actions in
Hoard of the Dragon Queen. They didn't rely on heavy ranged firepower, because I used the above tactics to remove that aspect of combat from play. I playtested the encounter 6 times before running it and yeah, they lost, TPK, about 4 out of the 6 times I tested it, in nearly all cases, the ranged DPR wasn't able to play much of a role because the dragon shut them down using the tactics I detailed above. So was it deadly? Yeah, but not a 100% guarantee of a loss by a long shot. Anyway, they were able to emerge victorious. Here's how it went:
The two stealthiest members of the party (bard and ranger) got a surprise round after they used
pass without trace,
bless, and bardic inspiration to sneak up on the dragon through a side tunnel, yes, they beat its insanely high passive perception, thanks to bless and a bit of luck. They got an opening volley on the dragon then retreated back into the tunnel to avoid its breath weapon while the rest of the party charged in through another entrance, separated so as to prevent them from being wiped out with a single breath.