Just set up the encounter, provide a getaway route, and stop there.
It's on the players/PCs to figure out that retreat is a viable - and sometimes the best - option. If they don't, or refuse to, then let the clear-conscience slaughter commence.
More seriously, while individual characters die all the time I'm often truly amazed at just how resilient parties-as-a-whole can be - someone always has a "getaway car", or finds a way to survive, or (and these are always the best) finishes off the opponent when everyone else is down and out.
Maybe Tanky dies this time. Makes up for last time when that massive fireball killed the wizard but Tanky was tank enough to survive....
Either way, the party - and thus the campaign - keeps going.