Ice/Lightning Mage

Thunder Spear

* 30 Thunder Spear or Element Composition (MAX)
* 11 Spell Booster (11)
* 30 Element Amplification (MAX)
* 30 Ice Strike (MAX)
* 30 Element Composition or Thunder Spear (MAX)
* 20 Seal (MAX)*(See Note Below)
* 2 Partial Resistance (2)

Ice Strike

* 30 Ice Strike (MAX)
* 11 Spell Booster (11)
* 30 Element Amplification (MAX)
* 30 Thunder Spear (MAX)
* 30 Composition (MAX)
* 20 Partial Resistance (MAX)

Element Composition

* 1 Composition or Thunder Spear (1)
* 3 Elemental Amplification (3)
* 29 Composition or Thunder Spear (MAX)
* 1 Elemental Amplification (4)
* 11 Magic Booster (11)
* 30 Thunder Spear or Ice Strike (MAX)
* 26 Elemental Amplification or Thunder Spear (MAX)
* 1 Partial Resistance (1)
* 1 Seal (1)
* 19 Partial Resistance or Seal (MAX)

Power Up First

* 5 Elemental Amplification (5)
* 11 Magic Booster (11)
* 30 Thunder Spear (MAX)
* 30 Ice Strike (MAX)
* 9 Magic Booster (MAX)
* 20 Seal (MAX)*(See note below)
* 25 Elemental Amplification (MAX)
* 20 Partial Resistance (MAX)
* 1 Magic Armor (2)

There are no mobs weak to thunder between Clangs and Vikings, so Thunder Spear may not be suitable as your first attack skill

Consider whether you should put points into Partial Resistance or not, after all, there aren't many monsters with such elemental attacks. In any case, Seal is probably better for everything but Zakum, while Partial Resistance is useful if you do Zakum all day (since it can't be sealed). Though, it's not like you're going to try to train on Grims or anything, and for Vikings Ice Strike freezes them solid anyway.

* Note: Seal is a skill that can be useful, but it is more useful to Fire/Poison Mages. Since As an Ice/Lightning Mage you can freeze your enemies, making them unable to use magic attacks, why would you need seal. I advise leaving seal to 0.
http://strategywiki.org/wiki/MapleStory/Magician_Guide/Builds