This question already has an answer here:
The phrase "to break a butterfly on a wheel" is very evocative, but I can't bring myself to use it: I find the "wheel" too disturbing.
So: what are some good alternatives?
(For those who are unfamiliar with this expression, Brewer's Dictionary of Phrase and Fable defines it as "to employ superabundant effort in the accomplishment of a small matter". [link])