That's not a combat encounter. You're not supposed to fight them.
First of you can sneak around, pretty easily.
Second you can find sulfur flowers in another area and just throw at those turrets to disable them.
While this is true (and I usually go for the route "Disable them all rather than fighting) there are problems with how the whole thing is framed.
For one, if you are not supposed to fight them, it should be made less easy/convenient to just play with the LOS and shoot them down, as it is now. Totally immune to damage of most type? More HP? Whatever works, frankly.
Because right now exploiting LOS and taking few minutes to shoot them down is a perfectly viable (if inanely boring) solution and it that's by design... I'm not a fan.
Also, the "stealth" path (or any alternate way to circumvent or disable them) should be a little more "readable" and less messy... Given what a wide area these turrets cover with their cone of vision (well, the first one, mostly) it always feel like I'm somewhat exploiting the game when I simply outrun their LOS-indicator once spotted, etc.
The best/fastest/most effective path I found so far was using Astarion (or whatever good lockpicker you are bringing in your party) to jump down on the giant fungi on the left of the entrance, reach the bottom of the tower, picklock the door and disable every turret without doing a single fight by activating the sussur-fueled furnace... But if your are going through a more conventional path (entering from the first floor and moving down) it feels like dealing with the turrets could get pretty annoying pretty quickly.
