Don't ask yourself: "Can this fight kill them?" Ask instead: "If they fight this, then that, then this other thing before resting, will they feel the pressure?"
Build your day as a chain. A warband skirmish burns basic resources. A dangerous climb forces fatigue saves and movement choices. A magic-draining zone strips spell flexibility. Then the boss fight hits when they're already compromised. Each link in the chain targets different resources so nothing feels random - it feels inevitable.