Hew she has a will save of +16? That's immunity, a 17th level caster with a 28 in the appropiate attribute, would still only cause her to fail on a 1.
You say you toned her down? Remind me to never play a Paizio adventure.
Ha ha ha, yeah! They can be player-killers in the hands of cruel GMs, but if you play them in the spirit as they are written (which is hard online), the writers clearly give some possible routes that can be taken to neutralise foes, or perhaps even things up with creative role-playing. Given than on a forum game this approach does not work so well, I am careful to instead make sure that there is perhaps a slight rebalancing of some aspects of the adventures.
The current adventure, Hook Mountain, is being run by me only in parts. The actual adventure is a rare mess from Paizo. It was written by one of my favourite writers, but he messed up on the word-count and about 1/3rd had to be cut. The continuity of the adventure was completely lost. For example, he had originally written an early encounter with the sister of Xanesha, the snake lady you reference above, but that entire encounter aspect was cut.
Anyway, out of interest, here is the stop part of Xanesha's stat block.
Xanesha CR 10
Lamia matriarch sorcerer 2
Always Ce Large monstrous humanoid (shapechanger)
Init +6; Senses darkvision 60 ft., low-light vision; Listen +2, Spot +2
Defense AC 26, touch 16, flat-footed 20
(+1 armor, +1 deflection, +6 Dex, +9 natural, –1 size)
hp 142 (12d8+2d4+48)
Fort +11 (+13 against poison), Ref +15, Will +16
Immune mind-affecting effects; SR 18
In other words - an INSANELY tough foe. You should check out the Paizo forums. There is a forum for Rise of the Runelords, with a long-running obituary thread. Xanesha has been the death of many, many adventurers.
I quite like the idea in DnD that not all encounters should be balanced. I like that on occasion a foe is far too strong for a party, as long as you provide an escape route for them and actually reward the players for being wise enough to ascertain that they are out of their depths. Not everything should always be solvable by combat.
Glad you are following!