The only one that sounds fluent is a fist, because it happens to be one fist but it is not important or known exactly which one, and it doesn't make a difference. The fact that the boss has two fists doesn't matter.
"The fist" sounds like it's referring to some specific fist that we've already mentioned.
This is just rule #1: Use the indefinite article a or an only with a singular count noun whose specific identity is not known to the reader.
It's one fist, but the specific identity isn't known.