That's never happened to me even with a boss that was too powerful for me. Never been one hit killed unless I was on master difficulty.

I don't do kiting with my wizard. I have my tanks, potions/poisons (weakness against *insert element* helps), turn the enemy on each other, buff out my wizard or I just flat out run away. Monty Python has taught me well

I've had to run away only once with a boss who ripped me to shreds a few times, but I lasted a while each time and tried something new. In the end, I just couldn't kill him. I still think it was glorious that I had to run away and I'll be back for that bastard later