My favourite headshot [Harrow] build, and with few adjustments, it will probably be yours too.

This is a 0 FORMA build as I find levelling warframes beyond frustrating. But if you are willing to do so using a maxed [Blind Rage] and [Power Drift] in the exilus is optimal.
Such build allows you to do all the lovely red crit things all [Harrow] buids do meanwhile allowing space for the continuous headshot buff (with [Lasting Covenant]) if you are good enough.

I personally prefer to use [Corrosive Projection] but Enemy Radar can also be used.
Also, the use of Arcane Acceleration is recommended with all harrow builds as him being a primary/secondary wepon-based warframe, increasing the fire rate automatically increases the DPS.

The other arcane slot can be used for any kit gun arcane or Arcane Grace / Arcane Energize if you are struggling with health/energy.

For my playstyle, Natural Talent is necessary as all of Harrow's abilities have a long cast in my opinion - allows for spamming of abilities (mainly 1).
Always have your Thurible charged as it is your way to have near infinite energy throughout the mission allowing you to spam Condemn or any helminth ability (Shuriken or Gloom are fun).

Love running [Opticor Vandal] (Crit) and [Zymos] with this build, allows landing consistent headshots with Condemn and keeps em red crits comin.

GL with the headshots