Yup, if the enemy is more than 5-6 levels ahead of you, you more or less have a 0% chance of defeating it. After a couple bad experiences I always check the recommended quests level before starting anything.Finally got the game on Saturday and played a few hours yesterday after the 8GiB of updates downloaded Saturday night.
So the first thing I've learned is, when the game says I'm not at a high enough level for some monster/ghoul/bandit that I should just run right then. Don't delay just run. Oh, and the chick has a nice ###.
To give an update on my play through, I've now been exclusively playing this game for a couple months (which because of kids doesn't mean a lot in terms of total hours) and I just exhausted the Velen/Novigrad quests and undiscovered locations I'm qualified to take on (level 23). got to Skellige and was pretty surprised to see that the map area there is almost as big as Velen. I'll be playing this one for a while.