How to build Unkillable Magetank (Respec at lvl 12+; * a is required skill)

Spirit Tree:
- Barrier *
- Gaurdian Spirit (Optional)
- Mind Blast (Optional)
- Strength of Spirits (Optional)

Knight Enchanter Specialization
- Spirit Blade (+ Upgrade: Defending Blade) *****
- Combat Clarity *
- Fade Shield ******
- Fade Cloak *
- Veiled Riposte ***
- Knight-Protector ***

Preferred Elemental Tree (Pyromancer is my first pick, due to Clean Burn)
- Immolate
- Flashpoint
- Clean Burn
- Immolate Upgrade (Wildfire)

After getting the mandatory Knight Enchanter and Barrier skills, and then getting the Pyro tree (or whatever tree of your choice, but I will expain why Pyro is super-duper-uber broken), work on getting the optional skills from Spirit Tree and Knight-Enchanter. It will make you very, very, very difficult to kill. After getting whatever you want from your elemental tree, go back and get Strength of Spirits, it will make your barrier last longer. Stay away from the left side of the tree, as you want to attract as much enemy attention as possible.

How this build works:

Right before entering combat, activate a Barrier on Vivienne, and run straight into the crowd. As soon as you are in melee range, begin to use Spirit Blade. This will do a LOT of damage, and it has no cooldown, so bind it to a spammable key. If you specialized in Clean Burn, you'll notice that your Immolate cooldown is actually 3-4 seconds (Since Spirit Blade counts as a spell!), so make sure to spam that too - especially since it will be taking half of your barrier to do MASSIVE damage, thanks to Chaotic Focus. When you hit your opponents, you take 30% of the damage dealt, and add it onto your barrier (thanks to Fade Shield). This effectively makes your barrier constantly regenerating, as you should be casting Immolate on long range targets when possible, and constantly spamming spirit blade in melee range. Combat Clarity means that you will ALWAYS have the mana to spam both Immolate and Spirit Blade, making you effectively a warrior with regenerating gaurd and a long range fire spell. Knight Protector and Strength of Spirits simply make your barrier last longer, so the downtime between you switching targets makes almost no difference on how fast it drops. Lastly, whenever you have a barrier active, 20% of the damage you take is REFLECTED back at the opponent.

Recommended Equipment:
Unlock Emprys De Leon, and then farm Silverite, until you get a Fade-Touched silverite. Add this to your best HEAVY armour made out of Silverite. This will allow your mage to have a Tier 3 Heavy Armour, as well as generate 5 gaurd per hit, so that just in the highly unlikely case that your barrier fails, you can facetank because you have full gaurd left.

I can do a writeup for Sera (Dual Blades) too, if you want!