I seriously doubt NPC spells are stronger then player spells.
I am running around with a 100 hp I would very much notice if their spells were 10X more deadly, if anything you are confusing the spells that are being casted and whether or not they are dual wielded.
Example:
Icy Spear + 25 damage
Icy Spear + all perks = 37ish damage
Icy Spear + Dual Wielded = 55ish damage
Icy Spear + all perks + Dual Wielded = 82ish damage
Then add in Ice Pike, nearly the same animation:
Ice Pike = 60 damage
Ice Pike + all perks = 90 damage
Ice Pike + Dual wielded = 132 damage
Ice Pike + all perks + Dual Wielded = 198 damage
Just look at their hands and asume the higher level spells when you are high level
Other spells that can be confused are Firebolt and Incinerate, Lightningbolt and Thunderbolt.
In the OP's case it sounds like he's talking about Frost Storm, which does damage every second you stand in it:
Frost Storm = 40 damage
Frost Storm + all perks = 60 damage
Frost Storm + Dual Wielded = 88 damage
Frost Storm + all perks + Dual Wielded = 132 damage
So how much health and resistance do you guys have.
I am on default difficulty, lvl 23 and all mages I see never dual wield a spell, its usually fire and ice or ice and lightning or lightning and the spell absorb shield that drains the casters magic while channeling it. At this lvl never seen an AoE spell that does dmg over time standing in it. I do notice however that enemy mages can channel that shield and never seem to stop. I drain my whole mana bar and they are still hitting me with firebolts while keeping the shield up.
So I did a test with tgm on and times it. 4min later the mage started using his dagger to hit me as he was out of mana. That is just insane.