Can duress be used on an assault charge and why?

There are two implications to your question which makes it
unclear, so I will address both.

Duress is not an element
in the crime of assault. It that were the case, every teacher who overloads a student
with work would be guilty of assault. To constitute an assault, there must be an offer
or threat to perform some physical violence upon the person
of another with the apparent ability to inflict that harm. Therefore, if one points a
weapon or even a clenched fist at another, one has most likely committed the offense.
Mere words alone, without the apparent ability or intent, cannot justify a charge of
assault.

As for duress as a defense to a charge of assault,
that doesn't work either. The only defense which would lie would be self defense, in
which instance there would have to be a very real tangible threat to one's physical
safety. Under any circumstances, self defense will not lie unless one uses no greater
force than that to which one is exposed.

Duress would most
likely constitute extortion or some similar offense; however, the mere act of causing
one mental discomfort without a physical presentation of violence would not constitute
assault.

The link below has excellent
information.