Mage is the Class that specialized in different strong-power kinds of magic (Actions that deal damage to all creatures (Ice Storm, Dawn’s Wrath or Fire Storm) or to one of them (Firebolt or Lightning Bolt) and good discipline and strategy in your army. You will be able to increase the power of the army every turn (the quantity after playing (Scouting Patrol or Imperial Reinforcements and the power by Ongoing effect of Divine Fervor and by buff from Fifth Legion Trainer). If you chose Intelligence and Willpower attributes you will play this Class.
You’ll be able to combine casting powerful AOEs like legendary Dawn’s Wrath or point removals like Piercing Javelin with keyword Prophecy and also use a lot of creatures with keyword Guard (cheap like Kvatch Soldier, average like Hive Defender or expensive like Auroran Sentry) to destroy enemies aggressive play. You can also use a lot of another powerful Prophecy cards (removals like Lightning Bolt, Guard Camlorn Sentinel or destroyer of enemy aggression Cloudrest Illusionist) and strong Legendaries (Miraak, Dragonborn – good for late game,Indoril Archmage – extra AOE, or Ayrenn's Chosen – great creature in decks where there are a lot of actions).
One of the most powerful and top decks now are Control Mage decks. For example:
There are a lot of good AOEs (Ice Storm or Dawn’s Wrath) or point actions (Firebolt or Lightning Bolt) which help you to control the board. Also you will play a lot of creatures with Guard (Kvatch Soldier, Hive Defender or Auroran Sentry) to destroy enemies aggressive play. You can also shackle your opponent’s creatures and then deal 3 damage to all of them after using combo Dres Tormentor+Winter’s Grasp or Shrieking Harpy. You can also play strong Legendaries (Indoril Archmage, Odahviing, Miraak, Dragonborn) and unique creature Manticora to control the board too. There are a lot of Prophecy cards in this deck (Piercing Javelin,which can destroy opponent’s creature, or Healing Potion, which can Heal yourself).
You can also build Midrange decks when you use this class. For example:
You can play a lot of creatures and give them all buffs(two Ongoing effects by Divine Fervor). You can also control the board playing combo Imprison + Imperial Reinforcements. You can sacrifice your own minions with Last Gasp (Cruel Firebloom + Brutal Ashlander).
If you play an aggressive game you should be able to build an Aggro Mage deck.
Dunmer Nightblade will give you an aggressive Item Steel Sword. You will also use creatures like Camlorn Heroto deal damage to your opponent. You will also use two Ongoing effects by Divine Fervor) to deal more damage by your creatures.
On Arena this class is one of the most popular because of strong unique summon bonus effects of the cards. But you must follow magicka curve and potential synergy between cards. Also you should draft actions and mass removals to control the board.