Interesting combo. Never thought of that. I see 2 possible ways to come out of this nightmare:
1) Permanent Control or enough Creature Control on hand or field.
That means your opponent has no target or no mindgate for this.
2) Hourglass (only Rainbow).
If you are able to play one card of your hand, you can use Hourglass (or Precognotion), and Nightmare is on your hand.
Hope, you don´t have another.
Edit:
Ok, it´s not a combo, because Nightmare and Mindgate are in the deck of different persons.