Q: what dose 'being a machine gun attack on a whole field of straw men' mean? 
The problem with this neat triad is that, while a tirade against horoscopy might be perfectly agreeable to most everyone (being a machine gun attack on a whole field of straw men  — who reads horoscopes save for feeble amusement?), assertions on life and climate are on another plane entirely.

Rex Murphy: Governor General appoints herself umpire of questions of faith and science
I googled and have understood the fallacy of a straw man, but still don't get the meaning of the bold phrase in the sentence above. What does it mean?

A: It's a metaphor.  Attacks on "horoscopy" (by which I assume they mean astrology, but that's not important) are usually pointless since it's such an easy target to criticize, analogous to shooting down a field of stationary targets with a machine gun.
A common English idiom is "like shooting fish in a barrel", meaning something is ridiculously easy.  The author tried to be clever by combining this metaphor with the "straw man" fallacy. 
