Here is the latest build I have worked on for a very durable mage.If anyone has already built something similar to this, no copying was intentional. I built it from a standpoint of immunities and caster level. Without further waiting, here it is; the Demonic Elemental.

Race: Human
Alignment: Neutral (this is a pvp orientated build so no smiting, plus elementals are neutral)

Stats:
STR 10 - 10
DEX 14 - 17 (AotS bonus)
CON 14 - 17 (AotS bonus)
INT 16 - 26 (all normal stat gains shown below)
WIS 12 - 12
CHA 10 - 10

Class progresson:
1- Wizard - Craft Armor 4, Concentration 4, Discipline 2, Lore 4, Spellcraft 4, Tumble 2 - Dodge, Combat Casting
2- Wizard - Concentration 1, Lore 1, Spellcraft 1
3- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1-Spell Focus-Evocation
4- Wizard - Concentration 1, Lore 1, Spellcraft 1-Int
5- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1 - Spell Penetration
6- Elemental Savant of Air - Concentration 1, Lore 1, Spellcraft 1 - Greater Spell Focus-Evocation
7- Elemental Savant of Air - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1
8- Elemental Savant of Air - Concentration 1, Lore 1, Spellcraft 1 - Int
9- Elemental Savant of Air - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1 - Greater Spell Penetration
10- Elemental Savant of Air - Concentration 1, Lore 1, Spellcraft 1
11- Elemental Savant of Air - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1
12- Elemental Savant of Air - Concentration 1, Lore 1, Spellcraft 1 - Toughness - Int
13- Elemental Savant of Air - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1
14- Elemental Savant of Air - Concentration 1, Lore 1, Spellcraft 1
15- Elemental Savant of Air - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Tumble 1 - Expertise
16- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1, Spot 9 - Int
17- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
18- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1 - Quicken Spell
19- Acolyte of the Skin - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
20- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1 - Empower Spell - Int
21- Acolyte of the Skin - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1 - Epic Spell Focus-Evocation
22- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1
23- Acolyte of the Skin - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
24- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1 - Epic Spell Penetration, Improved Combat Casting - Int
25- Acolyte of the Skin - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
26- Acolyte of the Skin - Concentration 1, Lore 1, Spellcraft 1
27- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1 - Automatic Quicken Spell I
28- Wizard - Concentration 1, Lore 1, Spellcraft 1 - Int
29- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
30- Wizard - Concentration 1, Lore 1, Spellcraft 1 - Automatic Quicken Spell I, Automatic Quicken Spell II
31- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
32- Wizard - Concentration 1, Lore 1, Spellcraft 1 - Int
33- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1 - Epic Skll Focus-Spellcraft
34- Wizard - Concentration 1, Lore 1, Spellcraft 1
35- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1 - Focused Spell Penetration-Evocation
36- Wizard - Concentration 1, Lore 1, Spellcraft 1 - Epic Skill Focus-Lore - Int
37- Wizard - Concentration 1, Discipline 1, Lore 1, Spellcraft 1, Spot 1, Tumble 1
38- Wizard - Concentration 1, Lore 1, Spellcraft 1
39- Wizard - Concentration 1, Discipline 1, Hide 20, Lore 1, Move Silently 20, Spellcraft 1, Spot 1 - Practiced Spellcaster (wizard)
40- Wizard - Craft Armor 2, Concentration 1, Hide 1, Lore 1, Move Silently1, Spellcraft 1 - Maximize Spell - Int

Final skills (not counting feat bonuses):
Concentration 43
Discipline 21
Hide 21
Lore 43
Move Silently 21
Spellcraft 43
Spot 21
Tumble 20

Final saves:
Fort 22
Reflex 19
Will 27

Here are the end results for character capability.
Pros:
Respectable max hps of 320 (minimum of 200) My build had 291 without rerolling for max.
Despite being only a level 20 wizard has a caster level of 39.
Immune to sneak attacks, critical hits, lightning, stuns, poison, sleep and paralysis.
All lightning spells have an additional +3 on spell penetration rolls, as well as +3 on DC checks
Has Darkvision.
Fire and cold resist of 20.
Fiendish Glare can give -2 on attacks, damage, and saves on failed will save of 20.
Glare of the pit does 16d6 damage once per day.
With epic lore focus, is capable of 7 epic spells per day, also with epic spellcraft focus and some gloves of epic spellcraft, has a +101 on rolls to learn and cast epic spells. That's high enough to learn every epic spell in the PRC.
Damage reduction of 20/+1.
+2 Natural Armor.
Pixie familiar can handle most locks and traps.

Cons:
Takes double damage from acid spells (fortunately has spell elemental immunity for that).
Even though decent hps for a mage, they are still low in general.
Can summon a vrock, but it's only 15th level.
AB is only +20 making touch and ranged touch attacks potentially tougher.

All in all I think this is a very solid spellcaster. Every piece of information given above was with the character completley naked.

Enjoy, critique, all that good stuff .