OP
Where to begin?
Well, firstly if you are playing as reloader you could in theory use almost any build...weak ones die easy of course, but so what, just reload and keep trying with variations in strategy until you succeed and save after every step forward....easy.
If you are looking at a "Dead is Dead" or "Death with Consequences" or "Death with minimum reloads" scenarios then you are going to need a survivor.
Difficulty selection is obviously a key issue.....the higher it is the more critical your precise skills choices at each level become.
However, the requirements change as you progress and you soon run out of perks and find youself being out paced by your enemies, until about level 50 where you start to have spare perks to place in less vital 'nice to haves'.
So....talking as from a Legendary difficulty perspective...cos that's my long term play on DiD with the Contessa
Early on defence is vital....attacks can be done piece meal....but you have to be able to sponge a hit.
A. Perk your chosen armour and smith it up as best you can....I go Light for speed in retreating!
B. Sprint from bears etc so keep stamina potions in stock always
C. Get a good shield and perk block with 1H for attacks in melee
D. Get a bow...and use poisons....To avoid contact if poss......stick a couple of perks on it....but not at the expense of armour and 1H.
E. Build Sneak to a good level. Hiding puts foes off and avoids dangerous fights
F. Grow your own paralyse poison ingredients.....best way to stop bears, cats and bandit chiefs
G. An flame atronach makes a good distraction during combat...fire it off first, then assist it.
Midgame...with good armour and 1H
A. Archery is less important...really invest in that 1H weapon now
B. Block is now vital since the stronger enemies need toe to toe bashing down
C. Learn to dance! "Tactical Evasive Manoeuvring" aka Don't get hit while dishing it out!
D. Make good use of shouts....esp..UF to blow foes off ledges. MfD to weaken dragons etc. and BE Defensive back stop
E. Poisons are less effective now compared to 1H attacks....just keep a few strong paralyse for toughies
F. Crafting skills should be getting high for powerful poisons/resistance potions and enchanted boosts and resistances.
Later game armour skill near maxed out along with 1h, block.
A. Start to incorporate a few magic tricks to get the edge with real bad dudes....Falmer Warmongers etc.
B. Build some magicka points and use spells like Runes, distant casting atronaches to outflank foes
C. With good armour resistances, a high AR of 700+, strong (90%+) resistance potions, 1h and bow Dmg 400+
You are ready to tackle level 50 Deathlords and tough Falmer
But you are not a god.....and Skyrim will pit increasing numbers of high level foes to overwhealm you.
So....comes the strategic requirement....
Recce ahead in sneak..use muffled boots/ invisibility to check foe locations (aura whisper is good here) ensure you have..
A.. The zone exit clearly noted
B. One or more likely safe zones to run to
C. A retreat plan just in case
Know your foe....
A. What are they weak to
B. What can't they do
C. Their speed, primary attack and how many can gang up on you in any area
Keep your primary objective clear in your focus....
If it is just to obtain something, then fighting may be unnecessary if sneaking will work!
If you desire to kill everything, leave the quest goal until they are ALL dead.....or you might get a nasty surprise afterwards.
This works for me (though I tend to avoid conjuration)
Other methodologies and builds will work, but if you want to go mage...I'd stay on Adept
The most vital piece of advice......stay adaptable and flexible....switch between disciplines to meet the tactical need
I am not an expert, and as I said, other approaches can work, and others may argue their favourites....but I specialise in long run Dead is Dead on legendary plays.....jus' saying
I'm done