OK, so Narcissa is BROKEN. In the originl build (before there were multiple classes and different skill trees) we weren't supposed to be able to beat her without losing. We were supposed to get our ass kicked and jailed, THEN get the Accelerator weapon... THEN beat her. If you beat her without losing, you'll get access to most of the scenes of her submission route.I grinded up another level but it feels tedious
1ST: the mechanics of the game currently allow you to cast Llavanya's Fury X2 for a huge attack bonus, then Top off your HP and MP at the Arcane Fire before going into battle. You can Save Spam for Excellent hits.
2ND: At LVL 12 you haven't gotten any OP'd perks for Ranger Class, I think they start popping in when you graduate to Master Ranger and choose a sub-class, BUT it seems like agility is high enough that Narcissa doesn't always get first strike, which is one of her biggest advantages in a non-Exploit fight.
3RD: Eploit Shadowbound=>Surrender=>Llavanya's Melody. Under almost all circumstances this is a game-breaking combo which completely prevents you from being defeated and often allows you an extra turn to attack or use a skill to get off some damage.
I beat her (1st Time) with a LVL 12 Ranger Equipped with Madeline's Song (for MP Regen) with Shadowbound, Llavanya's Melody and regular attacks. When I was caught in "Itsy Bitsy Spider" I NEVER wasted 75TP to break free when I need 100TP for Llavanya's Melody so I can recast Shadowbound on the next turn (and TBH, I really don't have a problem with Narcissa crushing my skull between her thighs ) There USED to be a mechanic in the game where you couldn't get the win over her if you had 0 HP even if you were Shadowbound, but it seems like that's been removed. I didn't even have to use Surrender. Pretty sure you could beat her the 2nd time with the same tactics.
Note: If you're still having trouble, you might want to advance your Corruption skill tree to get Katrina's Skill that induces bleeding. Bleeding takes off a significant PERCENTAGE of the enemy's HP, but won't kill... instead it leaves the enemy at 1HP so that even the most minimally effective attack can finish off the enemy. You can count on 2-4 turns of severe damage due to bleeding if the effect is successful, allowing you to do nothing but spam Shadowbound=->Surrender=>Llavanya's Melody while the enemy weakens.
The only Game Over-esque mechanic in play now is the Stress mechanic. As long as you don't get mind-broken... you can grind for levels, offer soul shards to reduce Stress or whatever.