My party was slightly stronger than yours at that point, but still not higher than lvl 20, I think. The Evil Eyes are the worst because they inflict insanity from afar. I just ran through that area with invisibility. From that part onwards, everything was much tougher. I did clear the Maze, though (full of Minotaurs), but not without resorting to copious save scumming. I also avoided places like the Land of the Giants, or any place where my characters could get utterly killed by a single blow, while I couldn't make more a dent to the enemies.
Oh, I just remembered. Fun fact about the maze--there are places where the big minotaurs will get stuck in the wall, so if you can manoeuvre them into those little spots, you can take out the headsmen and the kings, without them getting close enough to even touch you. The little ones can still move around and attack you, but once you've killed a few of those, you can level to the point where they aren't a threat anymore. As I remember, all those together should be enough experience to get you strong enough to take care of the hydras.
It's also true of Thunderfist Mountain, but it's much much harder to do.