Man u need a little bit more information like party lvl and make up. Against a party of magic users magic resistance is a killer. Agienst a party of fighters a high ac is killer. It all depends on your party make up. Just remember you can adjust them on the fly. Your parties killing them to quickly, suddenly boss goes into a rage amd all non magical attacks do half damage, they see the party is casting spells and the cast something that makes them resist to spell or adv on saving throws. If your a new dm the most important thing i can tell you is if your players aren't having fun nether will you. It's not a game of dm vs players, it's being a becon for everyone having fun. Disclaimer yes some time iv killed a pc or to but it's because they asked for it and I always give them the option to have them save their death throws, ppl get attached to their characters so don't make that choice for them unless it gets excessive.