I think you are right, I was going to make it stiff at first, but then I was checking some predator kills, and noticed how loose they had it. Maybe it's not totally realistic, but I think it'll do.
I had the skull showing more clean spots, but wasn't really feeling it. I can show a render if you want.
About making animations, I go in this general order.
Start with reference, better if it's video. Make a gif of the MK sprites or record yourself doing the motion with a camera.
1 = Block out the motions with a low frame rate. Let's say if you had video playing at 15 to 25fps, so larger gaps between frames. Think of these as the key poses with hand drawn animation. Make all the keys for a Key pose on the same frame for now, so you can edit them all at once and not be confused if they were spread out. Near the end of the animation, is where you off set the keys and call it finished.
I put my key frames at every 3 to 10 frames, loosely. Depends how different the pose is from one to the next. If you can get away with a longer gap, do so.
2 = Set the movement option in the time line to snap to key frames. Don't go one frame at a time, just skip from key pose to key pose, and see if the animation is starting to feel right.
3 = If that's looking OK, make a test render and see where the transition animations are looking strange.
I use the test render, because my computer isn't fast enough to play it smoothly in the viewport.
4 = Add Key frames between Key poses that have an odd looking transition. Closer to the left, center or right, depends on the timing you need, Slow in, steady or slow out.
Test render again, and see how that feels. Feel is very important, you just have to look at it and notice where something is off. Imagine the motion, think of a whip or a bouncing ball. Check your mind vs the render to get an idea of what to do next. You expect for it to move one way, but it's looking different.
5 = Keep making changes like that, trying not to add a lot of keys, only if you really need them. Get the animation looking as good as you can before moving to the curve editor.
6 = Near the end, mess around with the motion curves. Usually making the transition from one frame to the next look smoother, If the curve is looking too sharp like a Z, make it more of an S shape.
7 = Thinking of the whip again, parts that are near the end of a chain, tend to move last. This means, the part of the body generating the force moves first, then you get a whipping motion all the way to the point where it "snaps". A boxer drives his foot in to the ground, the energy moves up through the hips, to the shoulders, elbow and the fist. Use that to alter the pose and offsetting key frames.
8 = Don't keep anything still for too long, if you have a pause in the animation, at every 10 frames or so, jiggle the bones a little, so they are not exactly in the same spot. It'll make the character shift a bit, like you do in real life when standing still.
9 = Keep making small changes till the motion feels right, or you can't make it any better.