If you can find some zombies or trolls trapped behind one of those Aylied gates you can shoot through the big hole in the metal gate and train without them hitting back. Similar to the above this works well with Dread Zombies and Trolls since they regenerate health.
But my favorite method is to brew up some Damage Speed poisons. Then shoot them at some melee only opponent. Not a Dread Zombie because they are immune to poison. But a Troll or Orge works well. That will slow them down or lock them in place. Then you can use them for practice.
Reman Broder in Skingrad. If you can't find him at home, try Surillie Brother's vineyard right outside town.
I think Mr. Broder only trains you before he goes to work. That is to say in his home. I usually ask him for training in his home around 6:30 or 7:00 in the morning, as I recall.
Also, Honditar in Chorrol. ...
Honditar teaches Athletics not Marksmanship. He does carry a bow, but athletics is what he trains.