https://www.youtube.com/watch?v=Ct9O91l2HPU
If you are playing on PC like me with the recent addition of Lunastra you probably have atleast a small handful of Empress Styx weapons to craft, but quickly found out that Xeno'jiiva Horn never drops.
Well turns out it has a 66% drop rate chance off of breaking the head, and a very small chance off of body carves, so the best way to farm him is to break the head and leave, however this is not normally possible due to the limited nature of his quest. But whenever the quest "Keeper of the Otherworld" is available you can farm the horns with this method.
So take your favorite high DPS ranged weapon build and throw on some part breaker and go bully