All of us GMs have been there. We create an encounter that we think will be a cake walk for the players and they roll terribly. Either that or they split the party and neither group can help the other. The question then becomes, “How does the GM same the party?” because we all know the players will never, EVER run away when they really, really should. The encounter balancing system of any game is not perfect, but 5e is notoriously bad, and the GM has to make adjustments on the fly. These three rules can save the characters’ bacon.
1) Remove Enemies From the Battle
Not long ago I sent my players to Hell. They were second level and pretty much spent for the day, but I wanted them to have a memorable experience. So I made a number of easy fights against weak devils that were chained to the slaves the characters were trying to rescue. I made sure to describe the slaves as non-responsive, like they were just following orders and otherwise not conscious of their actions. So one of the players tried to rally the slaves and have them fight back. All that did was get the attention of ALL if the nearby devils, instantly turning several easy encounters into one very deadly encounter.
So how did I rescue the players? I took some pieces off the board. That infernal boar there? It no longer has 25 hp. It now has 10. That imp over there, it uses invisibility on itself to get into a better position and never pops back out. All of the sudden, that encounter became much more survivable.
What else can you do? Have an enemy run away. Have an enemy hide and then make an escape through a previously undiscovered tunnel.
2) Have Some Reason for the Battle Other Than Killing
Unless the players are there for the express purpose of killing the other side, there is some goal in mind. The same is true for the bad guys. Is there some MacGuffin the characters are trying to keep safe that the NPCs are trying to steal? Add a rogue with mage hand or a wizard with telekinesis to grab the item and the bad guys retreat. When the goal is no longer there, the fight is no longer important.
3) “Invite” Them to Leave
Those of you familiar with Critical Role know a character dies pretty early on in Campaign 2. It was obvious that that battle went against the players. Had it continued, it would have resulted in a TPK. Instead, the leader of the bad guys told them to leave, saying he’s teaching them a lesson. They left, licked their wounds, got more help, and then came back to fight.
That was definitely a loss, but inviting the players to leave took them out of the battle for a moment, letting them assess their situation and make the right call. Yes one character died, but the others lived and the story continued. Don’t be afraid to kill off one or two characters; be afraid of the TPK.