But how hard should an individual enemy be to kill. Should it take only people that have super enchanted armour and weapons that can kill it after a tough fought battle and if so then how many of this type of character should there be. You also could not have this super enemy in a cave or dungeon because not everybody will be wearing super enchanted armour and weapons to defeat it. So there does have to be a certain level of difficulty for the enemy so that they can be killed by all the different playing styles rather than just one smithed out enchanted one..
imo, no.
higher/est level enemies should only be able to be beaten by highly skilled, high level characters with the correct equipment and ability.
otherwise, run.
This is the classic conundrum faced by level designers since Day 1, and doesn't really have a satisfactory solution. On the one hand, if you make it so that the 'ultimate' challenge can be beaten by all then those who heavily tweak their characters will obliterate it, but if you
don't do that then said 'tweaks' are the only ones that can beat it, which kills build diversity if you actually want to participate in the end-game.
In the case of
Skyrim Bethesda has clearly chosen the former model, which isn't too surprising after the lambasting they received for their ham-handed implementation of level-scaling in
Oblivion. That doesn't mean it's a better choice, as witness the multitude of threads about the game being too easy once you have top-end gear, however the latter model only works if implemented properly, which is tricky due to how easy it is to overdo it.
I got a sample of that while working on my scaling overhaul mod for
Oblivion; while poking around and taking notes I discovered that the problem was not that things scaled, but rather that the modifiers given to the various stats of certain creatures and NPCs were way too high compared to the rest. Once I dialed these down, the 'arrow sink' problem disappeared completely save for a few bosses with high armor or shield spell ratings, and those were supposed to be a pain anyway. It took a while to get the settings right, though, as dialing them down too far made high-level combat a total joke. Prior to undertaking said overhaul I was adamantly opposed to scaling due to how insane some enemy HP pools got, however once I started tinkering with it I understood why they used it.
Obviously, the ideal would be a combination of both; some creatures/NPCs would be relatively static, others would scale to varying degrees, and yet others would more or less keep pace with the player so that top-end characters would still be able to find challenging fights. This begs the question, of course, as to the extent to which each would be employed, and that's where things get messy.