Well, the things about her that make me less nervous:
- She costs 5, and isn't in ramp colors (though bant). Hopefully you've done enough already that you're about to win or you already did. Honestly I'd rather they cast this than Sphinx's Revelation, though of course they could Sphinx end of my turn then this.
- After detain she's just 1 creature. Most R/x decks have swarm strategies. If you can afford to lose a creature each attack, you can race this. If they attack with it, that's good for us because it's turned sideways during our attack step.
Mono-Red obviously has the least tools here. But there is
Pyreheart Wolf and
Madcap Skills to force multi-blocks. Hopefully you can assign all damage to the other blocker and still take out one of their creatures. Or you're just getting around her entirely.
Once you start splashing other colors there are more
answers, especially in white and black.