Get Minion Instability if you do, might as well for more fire damage as well as Pyromaniac and the +burn damage nodes that go with it, then if you use ignite support gem w/ zombies + minion life you get a nice little dps boost.
You don't have to get all of the minion nodes either so you can still do good fire damage as a caster.
Build I was using
Soul Siphon is also good for casters, but you probably already know that
I'm not going for minions on my Shadow, I won't need them, all I need to worry about is not killing myself.Build:
at 79
Pluses:
-Eldritch Battery
-15% Cast Speed
-19% Spell, 20% Area, 20% Elemental, 18% Lightning, 68% Fire, 80% Burning damage.
-20% Chance to Shock when dealing lightning damage
-28% Increased Area of Effect
-44% Armour/ES
-20% All, 16% Chaos, 15% Fire, 15% Lightning Resist
-310% increased Spell Crit Chance
-150% increased Spell Crit Multiplier
Minuses:
-Eldritch Battery
-Only 150% increased Life
-Only 20% Mana Regen
-Only 8% increased Mana
Should be looking at around 3k Life but im gonna be stuck wearing Armor/ES gear just to have enough mana to support myself. Have a Searing Touch already, need a better one but ill figure something out.
Fireball w/ LMP+Added Lightning+Concentrated Effect+Life Leech
2x Fire Trap w/ Concentrated Effect+Increased Crit DMG