Take a talent in arrow redirection (freeing up a mutagen slot) and two in fortitude, giving you extra vigor, as soon as possible, with any other training skills being a matter of personal choice.
The end result build should look something like this.

Serious emphasis should be put on getting level 2 impregnation (alchemy tree) before applying ANY mutagens, as this build benefits more from this aspect than almost anything else.
Once this is done, ANY madness mutagens[witcher.wikia.com] should be applied wherever possible, as they add massive damage to signs in comparison to any other mutagens.


Alternatively, for a more madness mutagen reliant skill allocation, with even less HP and more damage, consult this tree, with a bigger branch into alchemy than the last, if only for the mutagenic slots.

Many will call me mad (that hurt) for the lack of points put into rolling, but if you're really dedicated to using signs and bombs in conjunction (with a minute amount of blocking) for the purposes of crowd control, it isn't quite as mad as you'd think, but is much more insane than you'd hope.

• Mage's trousers[witcher.wikia.com] are given to you if you've enabled the DLC. Take them to the end of the game, as they're the only mage oriented pants.
  
• Moonblade[witcher.wikia.com] can be brought over from Witcher 1 with a save transfer. Looks like those trophy quests come back to help again!
  
• Keep an eye out for the superb long sword[witcher.wikia.com] throughout the game if you don't bring a save over.

• Summer's Crown[witcher.wikia.com] can be used to buff your quen as you see fit.
  
• Light leather armor[witcher.wikia.com] provides a slight vigor regeneration buff.
  
• Shiadhal's armor[witcher.wikia.com] provides the best buff on sign damage for the chapter.
  
• Barber-surgeon's diagram[witcher.wikia.com] can be found in one choice towards the end of the chapter.
  
• Supposedly a sword called Devil[witcher.wikia.com] is in the asylum.
  Highly questionable.

Depending on what path you're on, some gear may or may not be easy to get, or available.

• Mage's gloves[witcher.wikia.com] should be taken all the way to the end of the game due to being the only gloves to give sign damage buff.
  
• Zireael's armor[witcher.wikia.com] provides more sign damage bonus.
  
• Armor of Loc Muinne[witcher.wikia.com] gives yet more vigor regeneration in combat.
  
• First-rate Temerian sword[witcher.wikia.com] gives a slight buff to vigor regen.
  
• Blue meteorite sword[witcher.wikia.com] gives a +5 damage bonus to signs.
  
• Robust blue meteorite sword[witcher.wikia.com] provides +7 bonus damage to signs.

Depending on what path you're on, some gear may or may not be easy to get, or available.

• Winter's Shroud[witcher.wikia.com] can be used to buff aard, providing you have the dlc, as with your quen buff.
  
• Dearg Ruadhri[witcher.wikia.com] provides a +5 sign damage.
  
• Draug armor[witcher.wikia.com] provides +5 sign damage, and is otherwise a straight upgrade over the Dearg Ruadhri armor.
  
• Armor of Tir na Lia[witcher.wikia.com] awards a massive vigor regen buff.
  
• Armor of Ysgith[witcher.wikia.com] is the single best mage armor in the game, awarding an entire extra vigor, +15% vigor regen in combat, and +8 sign damage. Be careful not to replace it with Vran armor if you're built for mage-work.
  
• Koviri sword[witcher.wikia.com] provides a +7% vigor regen buff.
  
• Novigradan sword[witcher.wikia.com] awards +10% vigor regen buff in combat.
  
• Forgotten Vran sword[witcher.wikia.com] is the best steel sword a mage-build can have, as it adds an entire vigor, something only one piece of armor does otherwise in the game.
  
• Super blue meteorite[witcher.wikia.com] sword is a +10 sign damage buff.
  
• Zerrikanterment[witcher.wikia.com] is the best silver sword a mage can have, as it awards +10 sign damage along with overall massive improvements over the super blue meteorite listed above.

By the end of the game, best case scenario, Geralt should probably look something like this.

• Arachas trophy[witcher.wikia.com] gives +2 sign damage.
  
• Elthon's trophy[witcher.wikia.com] gives +1 sign damage.
  
• Werewolf trophy[witcher.wikia.com] gives +1 sign damage.
  
• Elemental trophy[witcher.wikia.com] gives the biggest sign damage, +5.

For easy reference, a list of potions that focus on mage-buffing.

• Lapwing[witcher.wikia.com]
  
• Maribor forest[witcher.wikia.com], key to aquiring ten (yes, ten) vigor for those last few fights, for which there is no symbol in the HUD.
  
• Tawny owl[witcher.wikia.com]
  
• Wolf[witcher.wikia.com], for aard knockdown and igni incineration.
  
• Petri's philter[witcher.wikia.com]
  
• Stammelford's philtre[witcher.wikia.com]

The last two, the combination of petri and stammelford's philtres, end up turning Geralt into a very low health, very immense igni focused mage that can't take much damage.

Prepare to spam igni a lot, and rely more on quen and grenades to resist damage and being swarmed than anything else.
Dragon's dream[witcher.wikia.com] can help make the most of each and every blast early on.
Being resourceful is, like any alchemy bending build, just as important as being skillful.
Know which enemies can be trapped in yrden, which enemies are vulnerable to knockdown, and learn to crowd control!
Be prepared to run sooner than usual as your quen gets hit, and know the arena you're fighting in.
Whenever possible, have an escape route to recouperate, and bring any delaying bombs you can to help aid your survival.
Don't be afraid to compensate for mage-oriented potions' downsides. If you go into a fight with half the health you usually do, and you keep dying because running out of the way of damaging hits isn't an option, you should probably take something to buff your vitality.

WARNING: SPOILERS FOLLOW