[Quoth]

Morgaine Chronicles, though she has a side-kick. Technically SF, but more like Fantasy & the Red Sonya sort of thing.

Edit:
Maybe check out the Janny Wurts "Daughter of Empire" and related sister series to Fiest's Midkemia / Riftwar.

Teresa Edgerton's "Mask and Dagger Series"
L.E. Modesitt Jr.: The Soprano Sorceress (Spellsong cycle)
J.V. Jones: The Barbed Coil
Humour alt reality Fantasy: Jennifer is "The last Dragonslayer" (series of 4) Jasper Fforde
Winterling, Summerkin & a 3rd (bit younger) Sarah Prineas.

Not really Sonya like and not much swords: Spinning Silver by Novik.
Female protagonist & Trilogy (related books after the Trilogy, IMO go off a bit) "The Magician's Guild, Trudi Canavan

My own Celtic Otherworld series has some serious Enchantment and women you'd not want to cross, though only some are human. Not at all like Red Sonya. Eilis (Alice) and Marion are particularly tricky. Eilis has no need of swords and Marion later decides automatic pistols work on magical creatures.