As it has a 5-yard range, Lash of Pain is basically an extra melee attack for the Succubus. It can crit (and seems to do so more often when the Succubus attacks from invisibility) and can be resisted.
This ability is usually best set to auto-cast; the AI will just use it on the Succubus's targeted enemy as often as possible.
Casting Curse of the Elements on the target will increase damage done. The amount of damage done by the spell is proportional to the +spell damage gear of the warlock and will benefit from the school of magic with the highest +damage.