Any advice on running a mirror match combat encounter?
I'm a new dm and decently experienced player but I'm a bit stumped on combat encounters and I want this one to be really challenging and fun. My combat encounters so far have not been challenging enough to make the players feel as if there was a chance they couldn't win and they've asked about having harder combat encounters. The party is 5-6 people (A rouge, paladin, bard, warlock, and monk as well as a cleric NPC character who a friend will be playing to try out the game. Also every player is pretty new to DND but have gotten a decent hang of the game) at level 4 who are in a changeling storyline as of the previous session. I want to set up a mirror match of changelings plus a boss monster but I haven't decided on how to do it where it's tough but still winnable. I thought about using their character sheets against them for the changelings but if I just did that with an extra enemy, being the boss, would throw off the balance and I think it'd be too much. Otherwise I could make statblocks for the changelings but I'm not the most experienced in that department and I don't want it to be too easy or way too hard. Any advice is appreciated!