Guidelines
1. Player Skill > Character Builds
Sessions are not a test of how well-optimized a character is. They are about creative problem solving.
2. Tactics
Fair fights hurt. Set the expectation (gently at first) that going toe-to-toe with monsters without a plan is unwise.
3. Low Fantasy, Not Grimdark Fantasy
Signal when danger is afoot, giving players a chance to react before inflicting harm.
4. Alternatives
Just because the door is guarded by a ferocious hell hound does not mean that the party needs to fight their way through. Let the players try sneaking past the dog, luring the dog away, or talking to the dog. Spring a fight only when an attempt fails.
5. Grounding
Be judicious when allowing external material. Bags of Holding and Goodberries trivialize survival.