I have never specifically farmed for it - the two I got where just lucky while I was farming or doing monoliths for other things. I dont tend to farm specifically for uniques in LE - its pretty hard to up the odds of finding certain items so I dont bother and just let Lady Luck have her day. The only one I ever farmed for was The Maw axe with LP, but doing Orobyss is an easy farm.
Regarding the Lightless Arbour fight - there are ways around the boulders - see Heavys video for some tips: