My party is very good at combat since they just kill whatever is in front of them that looks at them wrong. I want to try and make an "encounter" that's harder than any fight they've had previously, but i want the major win condition to be something other than just murdering their enemies. The potential encounter i'm setting up doesn't start with hostiles but i'm sure my party will attack them before asking questions.
I'm a first time DM so i really don't know how to go about this properly but my idea was basically to have the combat be super hard and have my party have to try to talk their way out or make a run for it. Victory in this instance would be more just surviving instead of winning, but i don't know if this would be acceptable or if it would be an uber dick move or what.
Are these lines of thinking something that should be done or avoided?