A stiletto would be perfect. Long, straight... An awl maybe? Something with a wider tip is generally better if it can be sharpened to a very fine tip (e.g. a narrow flat-head screwdriver, with the head not only sharpened to razor thin but also shaped like a leaf with a slightly rounded tip).
You could use any knife that's very long and thin bladed though not too bendy.
The method usually involves a killing cone. Take the head in one hand, thumb and two fingers under the chin, index and middle finger on either side of the comb, beak sticking out between them. Turn the head a little, and use thumb and little finger to open the beak.
The blade goes up at an angle through the groove in the roof of the mouth, toward the point where the head meets the rear of the comb. Get the aim right without pressure, then use a sudden sharp jab. About the same pressure as... well it isn't that much. The bone is thin there, like firm eggshell. A jab, then a quick twist and it's done. The cranial skull is much stronger, so I doubt you'll meet your hand (but you could always wear a leather glove).
It's hard to describe! And a little ghastly to do, to be honest. But I do believe it's quick.
The only thing I'm not totally sure about is the exact aim the back of the comb seemed to work for me but perhaps someone who does it regularly and has a better description could say?
I haven't used this method in a long time, so feel I'm pretty rusty... I hope you get some clearer answers!
regards
Erica
Edited to clarify.