Swamped

If you don't have any other ideas, stall for time.

"And where exactly are you planning to go? It's a big ship, after all. It'd take you a while to get to any possible exit. Plenty of time for us to figure out how to get the mask away from you safely."

"I have my own way out," she replies calmly. "Not that it really matters."

"Maybe you do. But what about the mask? In its current state, casting a spell on it isn't safe. Or are you telling me this exit of yours isn't magical?"

That gives her pause.

"What do you mean, not safe?"

Another chicken appears.

"You know it yourself. That thing's filled with unstable ether. If you cast another spell on it, who knows how the ether would react? If you're lucky, they cancel out, but then, I have a feeling the ether's the main thing about the mask that interests you."

She doesn't say anything. She must be worried that if she does, you'll figure something out, something she doesn't want you knowing.

Then, suddenly, she laughs.

"You're right. It isn't safe to use spells on the mask. But that's not a problem for my knife here."

She lets go of the knife, and Donnie. Then the knife floats to his neck.

"So now my hands are free. And that knife will go for his throat at the slightest provocation. Don't bother trying to dispel the effect, either; if it detects you casting anything, it'll move in an instant."

Damn. Playing for time worked to her advantage, not yours. She picks up the breather mask.

"Now I'll be taking this, and walking out of here until I find a better place to leave from. And the knife won't drop until I'm safely off the ship. I'd recommend you stay out of my way."

She's starting to walk to the exit. If you don't act now, you may not get another chance. But what can you even do?