Chasm Stronghold - Sandy Canyon Outside the Great Hall

After escaping from the Stormguard great hall, follow the canyon to the east, picking up some shotgun shells on the way.

As you approach the timber-framed doorway, an Oni warrior drops down from above.

Two more warriors then run out of tunnel ahead. Shoot them as they approach. You may want to use the rifle here. They move fast so you may not have time to take down all three if you're using the bow.

They wear light armor and helmets, so it helps to concentrate fire on their heads or one part of their bodies to first eliminate the armor and then actually injure them. Or, aim for their legs, which are not so well protected.

Once you do a little damage, they'll fall to the ground, weakened, and you can either continue shooting from a distance or move in close and press the Melee button...

...to use one of your finishing moves. (Here it's the Rifle Expert finisher, which you may or may not have unlocked at this point.)

The results: A very dead undead warrior and mucho bonus XP.