I have an exercise I use with all my advanced beginner/intermediate skiers and it's the single biggest exercise for seeing large improvement. I call it the 'Grunting' exercise. All they have to do is grunt or make a loud noise towards the end of their turn. I don't explain why or what it's supposed to do at first. I love it as you literally can't get it wrong and nearly everyone improves.
Do you have a simple 'fix it all' exercise you commonly use?
You've probably figured out that my exercise helps them use their core muscles without realizing it, so whey they finish the turn they're more centered on their skis, there's less 'arm flapping' and other signs of an unbalanced skier.
There's very few skeirs of this level who couldn't do with a more balanced finish to their turn.