Michael
That's a good point. Plus, it's probably not a good idea to piss him off. Though, thinking of Castiel, it reminds me of another time I saved his miserable dying ass by siphoning another angel's Grace, slitting her throat and putting it in a vial and feeding it to him. That was back after Angelfall and Metatron's meddling. Yet another one of Castiel's fuck ups, of course, if he'd never trusted Metatron in the first place, well.
But my point being is that if you gave Lucifer the same throat slitting treatment, you could probably siphon some of his Grace easily. But I'm sure you're aware of such methods already?
Can't blame a girl for trying. And it wouldn't, but it would feel satisfying.