fortify also clears decay/disease and is very useful to have all the way through the game
the easiest part of the fight (especially if you have a rogue) is killing the witch with physical damage. if you have knockdown spells you can take her out before she can use any abilities. she'll always run up on a specific ledge so position a physical damage dealer up there beforehand
there is unfortunately no easy way to deal with the additional enemies she summons; you have an avatar as your main which i assume is a custom character, so that means you have access to dome of protection. don't be afraid to clump a few of your characters together under the dome, it helps a lot against the coagulated spit barrage each turn
play with knockdown and cc, the zombies have very little armor
