I've been starting to learn how to do fist fight type of stuff, lately. A couple of things I recommend are to do shaky cam and move with the characters movements some of the time. Also, when throwing the actual punches, use ease in and out, and make the movements flow. Make sure that the movements isn't too fast, nor to slow, as these will make it appear choppy or boring. And as Minifig051 said, if the camera angle is right, and you find it hard to make the characters actually come in contact, then don't have them touch. And, of course, make sure the minifigs are not too close or too far apart from each other, as this makes lots of issues when animating (I learned from experience). Now, Masterlego mentioned blue tack. I wouldn't suggest this, unless the shot doesn't include the feet, or if you use it without much being seen by the camera. Otherwise, you may be forced to mask it out, which means the shadows will get messed up and the shot will look too fake.
Hopefully you can understand some of my rambling here, and get some help from it.
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