Just completed G2:NOTR as a dexterity based mage! The last two chapters were incredibly easy with this build :) This post is mostly to help anyone who is thinking about playing as a mage, but is having difficulty choosing where to allocate their precious LP. Note that before Ch 5, I only put 5 LP into my one-handed skill, and raised it to 29% with tablets, and then trained it to 59% in Ch 5 with leftover learning points. Any comments or suggestions are welcome.
My endgame stats were:

Level 47
LP left: 51

Str: 59
Dex: 92
Mana: 426
HP: 941

One-handed: 59%
Two-handed: 32% (w/ tablets only)
Bow: 36% (w/ tablets only)
Crossbow: 26% (w/ tablets only)

Other Skills learned:
Pick Locks
Pickpocket
Alchemy (all mana-based potions)
Trophies
Language of the Builders lvl 3

Runes learned:
Fire Arrow
Ice Block
Ice Lance
Lightning (4th Circle)
Rain of Fire
Wave of Death
Create Zombie (claw of Beliar)
Holy Missile

Runes found:
Fear
Heal Light Wounds
Heal Medium Wounds
Create Goblin Skeleton

I was surprised how many learning points I had left over. If I had known how high my level was going to be, I would have spent the LP on Small Fire Storm in Ch 3 for better crowd control... I trained mana all the way up to 94 before using most of the stat boosters (tablets, potions, and 25 mana from giving Vatras Old Stone Tablets).

Anyone manage to get their mana higher?

That's serious mana. My DEX Mage games all had more DEX (and bow) and less mana. I can see how your build would rock at the end but the first two chapters would have been hard without a bow, at least the way I play.
DEX mage is the beast way.

In the early chapters, I spent a few LP on DEX and then raised it further with shrines, potions, and equipment. I managed to get it to 50 fairly early, then I ran behind the two trolls near Onar's farm and picked up the Rapier which became my primary weapon until I picked up the Master Sword. I was able to use fairly decent bows but it was more of a last resort ;) And yes this build was a lot more fun than my first playthrough as a merc!

I played merc the first time too. It was fun and it made sense as a continuation of the story from Gothic 1, but I tried mage the second time and I found it to be a lot more entertaining.

I don't know whether you've ever played Risen, but I also recommend playing as a mage in that game. In some ways it's a more fully realized version of playing a mage in Gothic 2.

Agreed!

I have played Risen, but only as a melee character and on the much inferior 360 version. Now that I own the PC version I do want to play as a mage, though I heard it's a bit underpowered.

Actually, I found magic to be best in the later game; I thought the multiple enemy melee was quite difficult.

I agree. The thing about magic in Risen is that the spells are utility based (like Levitation), not direct-damage combat attacks. Your direct damage magic comes from your crystal focus, which can be fire, ice, or pure magic (the bullet).

Most people seem to like fire, but I agree Magic Bullet is the best. Not only does it shoot super fast, it has a knockback effect to keep charging monsters away from you. Once you get enough levels in Magic Bullet nothing should ever manage to touch you again.

The great thing about mages in Risen is the whole monastery setting and all the quests. It's the most fun type of character to play for that reason.

The monastery was a major selling point for me wanting to play as a mage. Thanks for the suggestions!