For some of the cases where enemies are getting multiple turns in a row, using an S-Break (RT+D-pad) can be a good way to get yourself back on positive ground. If you get a turn, it also helps to give defensive buffs to the VIP before the enemy attacks them.

That said, I have heard that while Nightmare difficulty poses a great challenge to some masochists and optimizers, it's not a difficulty level the developers really balanced around.