You will need a full stack of Serpent Arrows, this was done with a Longbow and i cannot confirm if this will work the same with any other bow.

Make your way up to the bridge above the boss fog door, killing all the Redmane Soldiers i the area.

You will spot this brazier next to the wooden bridge.



You can hop up on to the wall & the brazier using the sacks in the back corner of the area.



You can manual aim just above the knights head and fire away.



This took about a full stack of Serpent Arrows to take it down. The last poison proc took about 20 arrows.

After that, hop down to the boss door & take you time with the Misbegotten Warrior.