There are key things to focus on regardless of what path you choose as a mage. Those paths being Dps, Hybrid and Healer.
The key stats for a mage in order of importance:
Matk - The main source of damage and causes higher heals.
Mdef/Pdef - Clearly help in survival. Mdef is actually more important for the Crypt runs while pdef is for against archers/knights.
Hp - Other then the obvious a higher hp on mages allows for Sunorita to heal you more per turn.
Troop Count - Is useful in allowing the troops to tank more for you. Get it where you can but don't aim for it.
Block - Is fairly useless. Blocking damage is always good but other stats are far more valuable.
Crit is useless as you'll want "Will Destroyer" which disables crits for amplified normal damage.
Patk - Is useless, I hope the reason is obvious.
Knowing that, the primary stats:
Strength = 4patk and 1pdef
Armor = 4mdef and 4pdef
Intellect = 4matk and 1mdef
Endurance = 20hp
Charisma = Troop count +1
In terms of skills there are certain ones you should be getting regardless of your chosen path:
Heart Elemental = Passive rage gain per attack
Mana Master = Passive Matk increase
Healing Empowerment = Passive Mdef increase
Lightning = Main attack and rage builder
I suggest the "Hybrid" build which focuses on healing but also allows damage output.
Assuming you max Heart/Mana immediatly
Lighting/RoF (Rain of fire) will be your main damage.
Restoration/Empowerment for single heals and mdef buff.
Sunorita should be gained and leveled to at least lvl 2 when possible for aoe heal and sustainability.
Your tree will look something like Lighting/Rof/Mana/Heart/Restoration/Empowerment/Sunorita - maxed asap.
***It is also important to note that the lvl 45 MP dungeon nearly requires a mage to have at least lvl 1 Purify so it is highly beneficial to get it by that level.***
As a healer you will want to focus:
Restoration/Empowerment/Sunorita
Getting Heart/Mana asap
Afterward you'll get Castinator, meteor, purify, blessed light.
Of course also leveling Lightning.
Your tree will look something like: Lightning/Meteor/Restoration/Empowerment/Sunorita/Mana/Heart/Castinator/Purify/Blessed.
DPS
As DPS you'll want to focus:
Lightning/Rof/Thunderer
Mana/Heart
Restoriation/Empowerment
You can also choose to get Meteoric or Damnation for a second aoe and extra damage output.
I discourage this class because I personally find it lacking so anyone that does specialize in this can feel free to make more in-depth analysis.
At some point I will add astrals and more detail but this is what I have atm.
Subject: Mage's beginner guide