You don't spec arcane for the scaling, you spec it for the bleed and the luck.
If worse comes to worst, there's always method I saw on Reddit that was ridiculous.
They went in with 2 incantations: a black flame AoE, and a huge heal. Summon the Mimic, then sit back until it's done. Ridiculous, and shouldn't work, but it does.
The black flame eats away at her health, and the Mimic never dies because its already large health pool (assuming a respec into health and faith) is supported by the heal which is AI driven.
But tbh, you can do this mate. Even if it means a bit of farming in somewhere you might not have been for some free levels (just ask, you can 100% go there).
I'll also restate a method I'm a fan of: spec into health and strength then use a Great Stars with Prayerful Strike on. It's a great hammer with bleed that heals on hitting, and the Ash heals you for an insane amount and floors her. Again, a Mimic could probably do well with that.
There's also the Moonveil route for INT builds (which will allow you to use Night spells that she won't dodge).
Rivers of Blood might still be the most brutal way to destroy her, because it gives her so little time to do anything if you get lucky and stun lock her with Mimic helping.