As Xbalanque kill an enemy who was stunned by your Ultimate.
Straight from the Wiki.
Focusing on Power Strength and cooldown should help you out here.
If an enemy stops moving during the ultimate they will not be stunned so be on the look out for that.
Arena is a good choice to attempt this but I personally prefer joust.
Edit- apparently the kill needs to be done before the ultimate is finished. That was never the case in the past but perhaps they changed it