A single
(only ONE) Photon with a frequency in the Extremely Low Frequency radio wave range.It’s frequency is too low to heat anything a noticeable amount (especially because it’s only one), and since it is only
one, it can’t cause any of the effects from being hit with lots of it over a long
or short time (the wikipedia page a couple of things they could do, but one single photon isn’t enough to cause them)
It cannot be shot at someone any faster than it usually goes, because it already goes the speed of light.
If the frequency is changed or more are added, it would no longer be a singular extremely low frequency photon, and it would be something else hurting the person.