Boss fights are WAY overtuned

Normal mode mage here - I went through and beat all optional champion bosses at a level deficit of 15 or more: changing up your whole weapon and armour set to match the vulnerability type of the bosses really matters until you have enough high level gear and skills unlocked to brute force a fight.  Also seek out the statues that give +100 health and the other ones that give you +1 skill point.  Check to see if you have accessories/armour that synergize with an element type.  I also found that I preferred orb and dagger combat over staff combat - but I would keep a staff of a different element on hand to cover any resistances any enemy might have until I got strong enough to brute force it.

The game tells you when you are about to get attacked - pay attention to the queues and pace your attacks so you’re not stuck in an attacking animation when you need to block.  Mage Rook dodges faster than they can block - you can animation cancel into a dodge but not into a block.  When in doubt just dodge.  If you’re against a single target, locking on makes this much easier as you can dodge behind them easier.  Rook gets spidey-sense when an attack is swinging in - you don’t have to block just dodge.  In coming AoE attacks are shown on the ground in red - don’t be there.  Unless you’re at the center of a dragon stomp you can dodge out of the way before it hits.  Dragons have a red glow/outline on their body when they’re about to lunge/swipe/kick - dodge.  For the lunge (I suck at this one) don’t commit to dodging until it’s already done its hop and has chosen a trajectory, then sprint and/or dodge out of the way.  If you find you’re getting swarmed Frost Nova is an invaluable tool - use it to escape and reposition.  The casting animation comes out fast and is on a cool down instead of costing mana which is useful when you want to save your mana for damaging spells or you are out of mana.  It freezes all enemies in place to give you time to reposition.  Aside from dragons - don’t bring it against dragons, it does nothing.  

Companions are unkillable - if you need to run until the enemy AI falls off of you and switches to them, then do so. The main bulk of their dps comes from setting up or detonating skills.  Bring companions that use attack types that the enemy doesn’t resist and can prime/detonate off of each other or Rooks abilities.  Aside from Darrin’s and Taash’s Taunt to take heat off of you I think Taash and Harding have skills that can make you invulnerable for a short period of time.  Bring someone, or two someone’s, who can force feed Rook healing potions.  Run around and dodge until those skills come off cool down if necessary.

Once you have runes for your the dagger use them.  I don’t know when each one is found but I think Besiege (for 20sec attacks vs barriers/armour isn’t penalized) and Ravage (for 20sec, +50% damage to health) are found early on.  Use besiege to strip armour and barriers, then ravage for a good damage boost to their naked health bar.  Even better if you make sure your hardest hitting spells are off cooldown and ready to cast once you proc ravage.

I retooled my entire skill tree from Evoker with points in Area/Control spell nodes and buffs to Staff combat (which I didn’t like) to Mourn Watcher with nodes for Duration/Area spells and buffs that would work with orb and dagger combat (which I found I prefer over staff combat).  I only did this once I noticed I had an armour set and weapon set that heavily buffed necrotic damage.  I’m at the point where it doesn’t matter if the enemy resists necrotic damage - my buffs completely over compensate.

My gameplay loop is dodge into the thickest group of enemies and Frost Nova.  As soon as the casting animation is done use an area spell that does DoT damage to the frozen targets and is a primer.  Use a companion ability that triggers a detonation.  Wail on everything until I’m ready to do it all over again.  Proc besiege when I see armour/barriers, proc ravage on red health bars.  My ranged attack (laser beam) is tooled to do more damage to armour/barriers too so I use that a lot.