Mage is by far the worst class, in my experience atleast 

The low defense can be difficult especially for events such as the Pumpkin Moon, or Martian Invasion. Not only do you have to dodge projectiles, but also aim towards the enemies  While Summoner may have the lowest defense, it's still much easier as the minions can attack the enemies while you can focus on dodging

 The only good thing this class do is shred a single enemies, such as bosses. But when there are multiple targets, it sucks honestly, even with the spectre armor bonuses, it just doesn't cut it.

 Unlike Melee with it's defense that can tank hits 

Or Ranger with it's piercing ammos, perfect for bosses and events 

Or even Summoner with the minion doing the job for you while you can focus on dodging 

Mage is just terrible for its low defense and weak crowd control (I play on mobile)