When they hit the bottom level of Thistletop, everyone in the party will be 4th level, and will include:
Human 3rd level Paladin of Sarenrae / 1st level Oracle (Ancestor)
Human 4th level Oracle (Black Blood/Spellscar)
She draws spells from the Sorcerer list instead of the Oracle list.
Half-Elven 4th level Spirit Summoner
Eidolon and he both build as Falchion-wielding warriors who fight side-by-side.
In addition, the party will include Shalelu rebuilt as a switch-hitting 4th level standard Ranger and possibly Orik as a 4th level standard Fighter, depending on how they deal with him.
Now barring unforseen events, I expect the PC's to first fight a group battle with a prepared Tsuto, Lyrie and Brauthazmus at the bottom of the stairwell leading to that level while Nualia looks on from down the corridor on the other side of the trap, observing the player's tactics. At what she considers an opportune moment she will send her Yeth Hound (who flies, remember, so it won't trigger the trap) to assist in the fight.
If the group defeats the bad guys decisively, Nualia will meet them in the corridor (especially if the trap splits the group up). If brought below half her hit points she has a strategy in place: she will flee down to where the Shadows lurk and cast Obscuring Mist, hoping to draw the Shadows out and the party in. She'll be subject to the shadows attacks as well but she has a Restoration spell saved for that purpose. Her plan will be to emerge once the PC's enter and attack any who try to escape the mists, possibly augmenting the Shadows battle with the PC's using channeled negative energy. As a last resort she casts Sanctuary and flees.
That's the plan, anyway. the thing is, I rebuilt her as a solid 6th level Warpriest (20 point buy, Strength and Evil Blessings) and, statted out, she looks pretty tough. With her buffs up, she looks something like this:
AC 19 (22 w/Shield of Faith)
HP 45 (57 with Sihedron Medallion's False Life)
w/Divine Favor, Bull’s Strength, Strength Surge, Power Attack:
+13 w/Flail for 1d8+15
+12 w/Claw for 1d6+7
Between her tactics (which I determined long before) and her statistics, do you think that's too deadly of a fight? I prefer to play my bosses smart and I want the fight to be memorable, but I abhor TPK's. What do you think?