Making a guard fire at an archery target is a bit trickier than I feel it should be but here we go.

Okay, lets jump into the CK.

1st

You need a guard equipped with a bow and at least one arrow. You will also need the archery target.

2nd

Position your guard directly in front of the target. You can adjust his distance later but about 12 feet away works. Below him, place a "patrolIdleMarker" and link him to it. It should looks something like this.

3rd

Guard

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keyword | Linked Ref

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| Patrol Idle Marker

Training Target | Archery Target

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For the guard's AI, copy "GuardSolitudeRangedTraining. Make sure to duplicate it before you start changing things.

Set the location for the cell he is in and set his Linked Reference for the Archery Target.

I wall update this later with screenshots. For a working model, look at my ColdHarbor mod. There a couple daedra training in the garrison courtyard.