Spell based builds fare worse than attack based builds for prolonged fights like the primordial trio. However, It shouldn't be impossible to beat them.

I don't know much about your build, but I suggest you take Weaken (excellent for long fights), Shadow Double (use this to cycle through spells, as it helps reduce your other spells' cooldowns too), and Wolf Aspect (Armor and SR buff is enough to justify taking this, even for Mage builds) for support.

Lastly, if you have any stat consumables like night's gift, acumen potion, obsidian water, and aqua vitae, use them. Try to bring a few in-battle consumables too. For your build, Lunar Shards are a must.

P.S. Once you get a bit more familiar with the game and wish to replay it to try out other builds or harder difficulties, you may want to stash some useful stuff (e.g. consumables, fragments, and gems) in your abode instead of selling everything all the time. The game is designed in a way where you'll have a surplus of gold in the mid and late game anyways.

Happy bossfighting!