I'm a new DM running a campaign for the first time, and I really want to have an encounter with a single, powerful enemy that the party must work together to overcome. Problem is, the party currently has six members, and I'm not sure what monster I could use for this. The book recommends a monster with Challenge Rating 2 for a group of six 1st-level PCs, but most of the CR 2 creatures can drop a level 1 PC in a single hit (often doing enough damage to kill them outright) I could use a CR 1 monster, but then I worry that the PCs will kill it before it gets a chance to do anything. I could fudge the monster's damage rolls so it always leaves the PCs one or two HPs from falling, but I feel like my players would catch on pretty quick if I did that. Any suggestions?