After you have saved the gnome mage from the remorhaz, buy improved invisibility from his spells tab (let him cast it on all of the party members) and you can charge head-on for Sherincal. She can't attack you including her minions because your invisible. She'll die like a roach. hehehe ;)

I discovered this as well. The only problem is ... that Improved Invisibility is bigged and any person "protected" by I.I. is essentially invincible. Enemies never attack a person protected by I.I., which is not what the II rules are supposed to do.

Regardless, the gnome has plenty of other potent buffing spells that will make the Shernical battle easier, such as Stoneskin.

Are you sure? In my game I let him cast I.I. on my monk and then rush in to the fight. Everyone can't see me EXCEPT Sherincal......lucky my monk has very high ac to miss most of her attack.....

It is a well known (by BIS) bug with Improved Invisibility.

When I last fought Shernical, I had the gnome cast II on my entire party. Then they walked straight up to Shernical and her pals and slaughtered them. It's the same thing anytime you use ImpInvis.

After wiping out Shernical with ImpInvis with my last party, I didn't use it any further. The bug is so imbalanced that it feels like cheating.

I think its not a bug because I.I. lets you attack, cast spells etc. and still not detected by your prey. Check the spell description.