I didn't quite get her to follow me up the hill but I love how she's just standing there with her back towards me while i turn the trebuchet.
I'd say the worst in nightmare is getting 100 approval in Wicked Hearts, Wicked Eyes. Your approval deteriorates during that battle and on the way to ballroom. It's a much longer fight in nightmare. I did it by bringing mages and having both me and cassandra as templars. Have a focus 3 mark of the rift ability for the first round of archers and demons and then dispell all the other rifts in the second round of demons. You should have enough time to run to the ballroom before it deteriorates to 99.
It helps to get well past 100 before doing the last part (approval counts past 100, it just doesn't show on the bar).
Also, as with all story missions on Nightmare, don't fight enemies if you can run past them. If they're guarding random chests, you can craft equipment that's better than what you'd loot.
My qunari inquisitor - despite his huge penalty to approval out of the gate - got Belle of the Ball on Nightmare by just running past groups of enemies unless he absolutely
had to fight them. Yes, that means he missed the locket and the naked guy, but that's for future playthroughs.