P.S. make sure you use a lightning staff to have every element at your disposal. (As Energy barage takes on the element of your staff. if you're not going with Knight Enchanter, you can replace Energy barage with chain lightning+Arcing Surge to circumvent this, though that does mean you'll still have to invest in Energy barage to get the passives, Conductive Current & Static Charge.)
These skills will be complementary to any specialization you choose.
Elemental Skills:
Storm:
Active
Energy barage
Passive
Conductive Current
Static Charge
Inferno:
Active
Immolate+Wildfire
Fire Mine+Searing Glyph
Passive
Flashpoint
Cleanburn
Chaotic Focus
Pyromancer
Winter:
Active
Fade Step+Frost Step
Winter's Grasp+Winter's Chill
Passive
Mana Surge
Frost Mastery
Winter Stillness
Spirit:
Active
Barrier+Elegant Defense
Mind Blast (Only to get the passive, don't use it)
Passive
Guardian Spirit
Strength Of Spirits
If you also want to be able to revive your party, replace fire mine with Revival+Lifeward
the focus skills in my opinion aren't worth a slot so don't invest in them.
Make sure to invest skill points in the inferno tree first as well as Fade Step+Frost step from the winter tree.
Anyways, I only played on nightmare and it was a breeze, though that isn't saying much. Again, it all depends on your play style and role-playing preferences.
Hope that helped.
Edit: Use Wrath Of Lovias for your weapon. Everite for Lightning element. Only use Great Bear hides when crafting it. Then upgrade the hilt and blade with the Hilted Staff Grip and the Dragon Staff Blade Schematics with Great Bear hide and whatever material enhances attack and critical dmg. It bumps your critical chance to 65% just for the weapon. My total crit chance is 86%. Also wear the belt of Elements and the superb Cooldown amulet (-15% Cooldown, eventually you'll get the Master Cooldown, which gives you -20% Cooldown from JOH). The Belt of Elements gives you 15% resistance against lightning, ice and fire. If you've done Solasan (20% resistance for every element) then it stacks to 35% total resistance for everything except spirit.
I used to craft the Staff of Corruption for the amazing attack boost but after a lot of testing they were almost evenly matched, though it was in favor of the Wrath of Lovias. Especially if you boost your crit damage and have high cirt chance like I do.