At a glance, here is the skill setup:
- Main Attack Skills
- Utility Skills
Standard advice for stats: Strength and Dexterity at minimum for equipment, Energy a few points if absolutely necessary to get by in the early game, Vitality for the rest.
Arguments for this skill setup (particularly the controversially low mastery levels) are below, in the section #Analysis.
You have two timered skills and two spammed skills. The combination of having two of each type allows you to maximize your damage regardless of typical monster immunities; the only problem you'll encounter is against Fire Immune Cold Immune monsters, which are all unique or super unique monsters (and are thus quite rare, occur only as individuals, and generally can be "parked" out of your way).
Against monsters that are not Fire Immune or Cold Immune, first cast Blizzard on approaching monsters, then spam Fire Ball. This is your main combination of spells. Blizzard stops opponents in their tracks and the Fire Ball deals significant damage in a small area.
Against Fire Immune, use Blizzard and Glacial Spike.
Against Cold Immune, use Meteor and Fire Ball.
When partied with a Necromancer or another build that needs significant number of corpses (a Magic Find Barbarian with Find Item, perhaps), you can use the Meteor-Fire Ball pair to leave corpses, and only use Glacial Spike in emergencies or against fire immunes.
Why the low Fire Mastery?Edit
At its first level, Fire Mastery provides a healthy +30% bonus to all Fire spell damage. At subsequent levels, however, it only increases damage by +7%.
Generally, you will mainly use only Fire Ball among your fire skills. Each additional point in Meteor gives a +14% bonus in damage. Thus, for your main attack spell, adding points in Meteor benefits your damage more.
You will only use two fire spells, Fire Ball and Meteor itself. When you upgrade level 19 Meteor (average 899.5 damage, ignoring synergies) to level 20 Meteor (average 987 damage, ignoring synergies), you get a 9.7% increase in damage, again better than the 7% increase promised by Fire Mastery. The relative percentage increase is even better when upgrading a level 1 Meteor to level 2.
Finally, +skills will increase the bonuses of Fire Mastery. However, +skills on Meteor will not increase the synergy on Fire Ball. It is thus more valuable to place points in Meteor, since low Fire Mastery bonus % damage can be fixed elsewhere.
Thus, for maximizing damage (and leaving further points for the cold spells), Fire Mastery should be kept at 1 point.
Why the low Cold Mastery?Edit
A similar argument to the above holds for Cold Mastery, but not as strictly.
Cold Mastery provides -20% resist for the first level, plus a subsequent -5% resist for each point.
When monsters have 0% resistance to cold, then minus resistances is exactly equivalent to +% damage. Your main cold spell, Blizzard, gains +5% also from Glacial Spike, but again this bonus can be increased with +skills in Cold Mastery but not in Glacial Spike, making it more vital to put actual hard points into Glacial Spike.
However, once monsters have non-zero resistance to cold, minus resistances is more powerful than +% damage. This tips the balance towards pumping Cold Mastery instead of Glacial Spike.
Generally, in the early game, monsters have 0 or even negative resistance to cold, so increasing Cold Mastery is not as vital. Later in the game, however, you should certainly consider increasing Cold Mastery instead of Glacial Spike.