You need 170% tri-elemental pierce. Build it like a normal caster and simply make sure your SF and tri-ele pierce is capped.
Some item choices include..:
Celestial Barrier
Celestial Barrier
Spiked Shield (Sacred)
Defense: (2525 - 2828) to (3012 - 3374)
Chance to Block: 1% + Class%
Required Level: 100
Required Dexterity: 550
Item Level: 120
+(4 to 5) to All Skills
30% Cast Speed
-50% Attack Speed
1% Base Block Chance
-(20 to 25)% to Enemy Elemental Resistances
+(150 to 180)% Enhanced Defense
Total Character Defense 50%
(0 to 2)% Reanimate as: Burning Titan
(0 to 2)% Reanimate as: Cruel Titan
(0 to 2)% Reanimate as: Frostborn Titan
Socketed (6)
Blackskull's Horns
Blackskull's Horns
Crown (Sacred)
Defense: 4437 to 4914
Required Level: 100
Required Strength: 606
Item Level: 120
+25% to Fire Spell Damage
-15% to Enemy Fire Resistance
+200% Enhanced Defense
15% to Strength
+50 to Strength
Maximum Fire Resist +2%
Socketed (4)
Fire Resist +30% (#s2)
Adds 100-150 fire damage (#s3)
+5 Fire Absorb (#s4)
Henchmen Trophies
(Legendary Set)
Doomcloud Spine
Doomcloud Spine
Plated Belt (Sacred)
Defense: 4089 to 4479
Required Level: 100
Required Strength: 606
Item Level: 120
+25% to Lightning Spell Damage
-15% to Enemy Lightning Resistance
+200% Enhanced Defense
15% to Vitality
+50 to Vitality
Maximum Lightning Resist +2%
Socketed (2)
Lightning Resist +30% (#s2)
Adds 100-150 lightning damage (#s3)
+5 Lightning Absorb (#s4)
Henchmen Trophies
(Legendary Set)
Fangskin Scales
Fangskin Scales
Greaves (Sacred)
Defense: 2481 to 2799
Required Level: 100
Required Strength: 595
Item Level: 120
(0 to 40)% Movement Speed
+25% to Poison Spell Damage
-15% to Enemy Poison Resistance
+200% Enhanced Defense
15% to Energy
+50 to Energy
Maximum Poison Resist +2%
Socketed (4)
Poison Resist +30% (#s2)
Adds 100-150 magic damage (#s3)
+5 Magic Absorb (#s4)
Henchmen Trophies
(Legendary Set)
Stareye's Claws
Stareye's Claws
Gauntlets (Sacred)
Defense: 1674 to 1959
Required Level: 100
Required Strength: 574
Item Level: 120
+25% to Cold Spell Damage
-15% to Enemy Cold Resistance
+200% Enhanced Defense
15% to Dexterity
+50 to Dexterity
Maximum Cold Resist +2%
Socketed (4)
Cold Resist +30% (#s2)
Adds 100-150 cold damage (#s3)
+5 Cold Absorb (#s4)
Henchmen Trophies
(Legendary Set)
Dementia
Dementia
Belt (Sacred)
Defense: (3111 - 3348) to (3272 - 3524)
Required Level: 100
Required Strength: 634
Item Level: 120
-(15 to 20)% to Enemy Elemental Resistances
+(15 to 25) to Fowl Fight
Slow Target 5%
+(120 to 140)% Enhanced Defense
+500 Defense
Maximum Life 5%
Maximum Elemental Resists +1%
Physical Resist 5%
Requirements +20%
Socketed (2)
Unveiling Eye
Unveiling Eye
Circlet (Sacred)
Defense: 505 to 571
Required Level: 130
Required Dexterity: 262
Item Level: 130
Hidden Reliquary
(Bonus per Hidden Crafts)
+25 Spell Focus
+20% to Elemental Spell Damage
+1 to All Skills
-(21 to 30)% to Enemy Elemental Resistances
Slain Monsters Rest in Peace
50% Magic Find
Socketed (4)
(And associated crafts)
The Tesseract
The Tesseract
Amulet
Required Level: 100
Item Level: 105
+2 to All Skills
25% Cast Speed
+(1 to 30)% to Fire Spell Damage
+(1 to 30)% to Lightning Spell Damage
+(1 to 30)% to Cold Spell Damage
+(1 to 30)% to Poison Spell Damage
-(1 to 20)% to Enemy Fire Resistance
-(1 to 20)% to Enemy Lightning Resistance
-(1 to 20)% to Enemy Cold Resistance
-(1 to 20)% to Enemy Poison Resistance
Natasha's Legacy (Xis)
Natasha's Legacy
Body Armors
'XisXisXisXisXisXis'
Runeword Level: 120
+(1 to 4) to All Skills
-(3 to 30)% to Enemy Elemental Resistances
+(322 to 350)% Enhanced Defense
Maximum Elemental Resists +(0 to 4)%
Physical Resist (5 to 20)%
Total Character Defense 100%
Cannot be Unsocketed
Robe of Steel
Robe of Steel
Full Plate Mail (Sacred)
Defense: (6692 - 7471) to (8202 - 9156)
Required Level: 100
Required Strength: 428
Item Level: 105
+(11 to 20)% Bonus to Spell Focus
25% Cast Speed
+(21 to 30)% to Spell Damage
+(115 to 140)% Enhanced Defense
+100 to Strength
+100 to Energy
Elemental Resists +20%
Total Character Defense 50%
+250 Life after each Demon Kill
Requirements -33%
Socketed (6)
[For SF capping if needed]
Wake of Destruction

Wake of Destruction
Greaves (4)
Defense: (90 - 102) to (135 - 152)
Required Level: 9
Required Strength: 60
Item Level: 16
+(4 to 6)% Bonus to Spell Focus
(0 to 40)% Movement Speed
+(6 to 8)% to Fire Spell Damage
+(6 to 8)% to Poison Spell Damage
+(11 to 13)% to Physical/Magic Spell Damage
+(24 to 40)% Enhanced Defense
+(21 to 25) to Energy
Requirements -20%
Socketed (1)
Defense: (333 - 369) to (462 - 512)
Required Level: 27
Required Strength: 84
Item Level: 32
+(7 to 9)% Bonus to Spell Focus
(0 to 40)% Movement Speed
+(10 to 12)% to Fire Spell Damage
+(10 to 12)% to Poison Spell Damage
+(15 to 17)% to Physical/Magic Spell Damage
+(58 to 75)% Enhanced Defense
+(26 to 30) to Energy
Requirements -25%
Socketed (2)
Defense: (854 - 930) to (1152 - 1254)
Required Level: 44
Required Strength: 164
Item Level: 51
+(10 to 12)% Bonus to Spell Focus
(0 to 40)% Movement Speed
+(14 to 16)% to Fire Spell Damage
+(14 to 16)% to Poison Spell Damage
+(19 to 21)% to Physical/Magic Spell Damage
+(92 to 109)% Enhanced Defense
+(31 to 35) to Energy
Requirements -30%
Socketed (3)
Defense: (1274 - 1378) to (1713 - 1853)
Required Level: 52
Required Strength: 292
Item Level: 86
+(13 to 15)% Bonus to Spell Focus
(0 to 40)% Movement Speed
+(18 to 20)% to Fire Spell Damage
+(18 to 20)% to Poison Spell Damage
+(23 to 25)% to Physical/Magic Spell Damage
+(109 to 126)% Enhanced Defense
+(36 to 40) to Energy
Requirements -40%
Socketed (4)
[For SF capping if needed]
Just pick what works and what you have and shoot for your SF and tri-ele caps. Then worry about defenses.