An impact wrench is the _worst_ way to do it for more than one reason....
I suggest you get yourself a decent blocking tool, or _carefully_ try one of the ways to improvise with different tools if you're confident about your skills with tools in general - there's plenty of guides out there on youtube etc...
Edit : beware : don't try the two stroke way of blocking the piston through the spark plug hole, for a four stroke engine, with its spark plug placed sideways, that method is even worse than using an impact wrench...