Pick all the units in the radius of attack, let's say 100 in this case.
From this group you will filter all the units that are not within the facing angle of your unit + something and the facing angle of your unit - something.
For example, if you want to be able to attack all the units like this \./ (the dot being the attacker), you will filter all the units that are not within facing angle + 35 and facing angle - 35.