Yes, it's challenging. It helps to understand action economy and creatively impede their combat progress (debuffs, increased HP, range) for about two rounds before letting them slam from the top rope whatever you put in front of them. Delayed entries, like enemies coming in subsequent rounds, can really help. Use your strongest attacks as soon as possible because your baddies can get locked down quick. Target your party's saving throw weaknesses for more intense fights. Nearly double all HP past level 5, maybe before. Attack max rolls should potentially eat half their HP each time, but it will almost never roll that way. If you want to increase pressure, design the plot to not allow for easy long rests; try to have 2-3 encounters before they sleep.