One of the things you have to remember is that is game, like most RPGs, has dice. Sometimes, the dice favor the players; sometimes the dice crush the players.
What you generally want to do is fudge things behind the scenes. Don't mess with the rolls or the encounter rules, lest your players feel betrayed or cheated. But you are the Narrator, and the entire story world is in your hands.
Fight looking too easy for the players? Call for reinforcements. Morale failure and scatter. Call a total defense action and offer the heroes information in exchange for being allowed to walk away. Take or reveal a hostage (NPC or hero). Change the stakes of the situation. Place the heroes into a moral quandary.
Or, just keep moving and make the next fight a little harder.
Fight going sour for the players? Instead of applying nudges to damage, break a weapon or shove characters into each other's way. Do something that opens opportunities for new ideas (What can the Lurcher do with a fragment of a sword blade?
). Don't take the heroes out of commission if you have any way to avoid it.
I watched a recorded RPG session where the GM brought in a over-powered group of villains. The heroes got pasted across the market square. Three of the four players dropped before two rounds were complete. Instead of finishing the hapless hero, the villains put up their weapons, told the survivor that "this was a warning" and "stay out of our way", and exited the field.
Almost certainly not what the GM had planned, but the adventure didn't end because of a bad fight. Instead, it became serious, and all the more epic. The players didn't turn back--they wanted revenge rather than just wages.