Also experienced this issue. Holding down the left/right buttons to adjust the turret appears to throw me out of the tank. If I did it in small increments, it was fine. The only other times I was thrown out of the tank was when a risen popped in close enough to hit me.
I took down three giants using the tank before I got fed up with being thrown out. I killed the remaining giants on foot (pretty much ignored all the other mobs). As soon the last one goes down (I counted six?), all the other risen mobs melt away and you can move on. I recommend anyone stuck on this section to just kill the giants on foot when they appear.