"As Cu Chulainn kill an enemy within 2 seconds of firing the Barbed Spear Ability and live."
Should be fairly easy to get this, aim for a weakened enemy and hit them with the Barbed Spear (Cu Chulainn's first ability) to finish them off, or hit them with a spear then attack quickly with a basic attack or another ability. Build the relic Aegis Amulet, or Bracer of Undoing which can also be of use, to help escape if you're in trouble, makes you immune to attacks for a few seconds.
Arena is likely to be best for this.