encounter Balance and general Combat tips
Hello
i'm still pretty new to DMing and i'm struggling with balancing my encounters. most seem too easy, even when i add some hp to enemies on the fly (i don't want to "cheat" too much).
I'd appreciate tips that go beyond "use CR or XP rating"
the current encounter that just started at the end of last session is as follows
3 players, 1 npc all 4 are lvl 4, Barbarian, Wild Magic Sorcerer, Ranger and a Rogue/monk (thats the npc, partially played by them or me).
they have encountered a Banshee and currently 4 Skeletons tho one died in round one by a crit from the Barbarian.
they are freshly rested but the Ranger has taken about 1/3 hp damage, because she was attacked by a shadow just before the fight and i rolled pretty well. i can add those into the fight if needed i'm thinking?
This is also a decently big moment in the current story, they are supposed to get to lvl 5 after this fight.
Setting is an abandoned, haunted Castle, cellar. all about a magical Amulet they are searching for. they found the amulet but that angered the Banshee.
so big questions i have here is, how would you make this encounter a bit more interesting/dangerous? how to make them more interesting/dangerous in general?
thank you in advance (feel free to ask me to clarify stuff if i forgot any information)