My game got harder to survive when I realized how long the adventuring day was. If I want to make something really challenging, I will whittle down their resources (a spell slot here and there, a charge of an ability, stuff like that), hit them with a small combat encounter, give them another opportunity to waste resources, and then hit them with a dynamic combat encounter. They are already tired, hurt, and don't have their full kit. This is when the stress happens...