Magic Users Guide to Dark Difficulty
First off, if your going to tackle the game on Dark Difficulty waving your sword around then best of luck and get use to the "You Have Died" screen. This game is ruthless, everything comes in packs and if it doesn't it'll most likely murder you in one hit. This is why choosing the magic path is a sensible solution to this hell mode, it allows you to keep your distances and when not an option, a means of evening the odds.
Aard - Your stun ability. It's a range attack that does little (if any) damage to a target but it will interrupt attacks and has a chance to stun, allowing you to instantly kill with your sword. This is not a very useful ability outside of chapter 1, the stun rate is low and most larger monsters are immune and its easily outclassed by a fullly upgraded Igni sign.
Igni - This is the main attack sign and your best friend. It starts of doing a moderate amount of damage to a single target but with the right talent points, runes and armor upgrades you can spam the hell out of it and murder large groups of monsters in seconds.
Quen - The second best spell in the game, it provides you with a protective barrier that will absorb an attack. With talents it can also transfer damage back to the attacker. The downside is you can not regenerate vigor while it's active, limiting your ability to cast other spells. It's best used when your forced to go sword to sword or in tight spaces, also works wonders against traps and fire.
Axii - This sign allows you to make allies out of your enemies for a few moments. It has a cast time that leaves you open to attack by others but once it is cast it will keep enemies distracted allowing you to get in behind and get a few stabs in. This ability is all about strategy and personal play style, there only a few parts in the game that require you to use it. It is best used in conjunction with Yrden.
Yrden - This is your main crowd control ability, it allows you to place a magical trap on the ground that will totally immobilize and enemy for about 10 seconds; attacks can cause the trap to break. This ability is best used with the Axii sign, allowing you to snag one monsters while you concentrate on hexing his friend. With proper talent placement Yrden turns into a force field that will prevent monsters from passing between multiple traps.
Vigor Regeneration (Training) - Basicly a must have, it increases your vigor regen while in combat.
Fortitude (Training) - More vigor combat regen, plus a second point in this talent gives you your first additions vigor bar and a mutagen slot.
Destructive Magic (Magic) - Increases sign damage and provides you with yet another vigor bar.
Enhanced Quen Sign (Magic) - Increases the length of time the spell lasts and gives the sign the ability to deflect damage back at your attacker.
Magic Intensification (Magic) - Increases the overall effect of your signs.
Magic Vigor (Magic) - More Vigor, more spells.
Enhanced Igni Sign (Magic) - Increases damage, range, adds an area effect component and a chance for targets to be set ablaze. A must have, ignore putting a second talent point into anything else until you put two in this one.
Control Over the Power (Magic) - Increases your sign damage plus adds passive resistances to all effects.
Schemer (Swordsmanship) - Increases your vigor combat regen by up to 40%. This is kind of optional, kind of not. I recommend getting this after everything else above.
Sense of Magic (Magic) - This gives you the ability to generate adrenaline which is used to trigger a slow-mo ability. I rarely used it, it takes awhile to generate adrenaline using spells and the ability only last 20 seconds and is limited to inside a small circle at the spot you cast it, it also a prereq for Control Over the Power so you have to put at least one point in it.
Enhanced Axii Sign (Magic) - Increases the health and damage done by mind controlled monsters. This is a must for anyone who enjoys watching enemies beat each other up.
Fatal Attractions (Magic) - Allows you to ensnare multiple enemies (up to 3) with Axii.
Enhanced Yrden Sign (Magic) - Allows you to place multiple (up to 3) magic traps on the ground. Incredibly powerful when combined with Glyph Enhancement.
Glyph Enhancement (Magic) - Allows Yrden traps to be tethered together, creating a wall monsters can't move through. Perfect for Axii users, no worrying about enemies hitting you during casting.
Alchemist (Alchemy) - Increases damage done with bombs. I found bombs useful while your waiting for your vigor to regenerate.
Equipment and Potions
I am not going to list out everything, there just way to much mage friendly equipment out there but I will give some tips on what to look for. First look for swords with multiple improvement slots, each slot can be fitted with a moon rune that will increase sign damage by 4. Their is a bunch of armor that has increased sign damage stats, the obvious stuff says Mage in the name but the best is found through quests or just exploring. Their is also a few armor improvements that will add sign damage but they can't be bought or created so you should save them for a piece of armor your sure you'll keep.
I rarely used potions, they require you to prep ahead of combat but I found that you move so quickly through the game that you just forgo their use. Another problem is most potions come with side effects, magic friendly potions will cost you vitality which can be the difference between two hits or one before you die. If you must use potions I recommend Petri's Philter, it adds +7 damage to signs with no side effects.
If your good at saving money I would highly recommend building the suits of armor available in Dark Difficulty. They are the most powerful pieces of armor in the game and while it is possible to put together a superior set of magic friendly armor, the rounded out stats of the dark armor makes them ideal for those encounters that magic might not easy solve.
Prelude Strategies -
The largest problem for the first part of the game is the fact that your abilities are so weak. You'll need to rely on everything in the prelude, not just magic. When fighting multiple enemy I would recommend using Axii to charm an enemy then while they distracted take a few cheap shots at the non-charmed enemy or deal any extras if dealing with 3 opponents. If dealing with more then 3, charm one to remove 2 out of the fight, then thrown a Yrden to trap remove another.
When dealing with the start of the "WTF am I suppose to do" dragon part, use Quen to protect yourself from the flames and use Axii to let the guards kill each other.
Chapter 1 Strategies -
Learn how to build Grapeshot bombs from Cedric, they'll supplement your damage and give you something to do while your vigor regenerates; you also need them to deal with the Nekker Nests, both in blowing the nest up and dealing with the swarms of Nekkers that spawn at them.
If you choose to deal with the haunted hospital I found that this strategy works the best; wraiths always spawn in pairs so place Yrden traps to capture one of them while you Axii the other, while they beat each other up place another Yrden sign down so when the charm finally breaks you won't have to worry about getting blasted by the other wraith during an Axii attempt.
When you fight the Kayan, I would recommend putting up your Quen sign. One hit from this monster will normally kill you.
When you fight Letho I would recommend attacking with Aard to begin with followed by sword attacks until he blocks then hit him with Aard again, when you run out of vigor roll back and just dodge his attacks as best you can until your vigor is back up. You can also try and throw grapeshot at him but do so at a distance because he will sword swipe you before you can throw if you're to close.
Chapter 2 Strategies -
At this point you should have the Enhanced Igni talent which means you can just spam the hell out of it and everything will die. There really isn't any notable battles in this chapter that require any sign related special strategies, just remember to Quen up if you have to go in with swords.
Chapter 3 Strategies -
Fireball Spam everything.
You will fight the dragon in this chapter and if your feeling ballsy you can try to fireball spam it but I would recommend keeping Quen up at all times (the dragon can one shot you and Quen will also protect you from it flame) and use grapeshot to bring its health down.
You can fight Letho again if you choose but this time you have a secret weapon, FIREBALL SPAM!
As an additional strategy to the FIREBALL SPAM everything you can use the combination of Yrden and Axii to take down almost anything. If both signs talents maxed out you can use Yrden to create a force field around yourself or across a narrow corridor while you use Axii to charm and quickly dispatch multiple enemy. It perfect in tight areas with multiple enemies, it also fun to watch.