Culling Strike is a property that causes hits to kill enemies left with X% of maximum life or less.
A hit with culling strike will kill an enemy if it is still alive with X% of maximum life or less after the hit. This occurs instantly; there is no delay between the enemy taking damage from the hit and being killed by culling strike. Culling strike work strictly off life percentages; energy shield, guard skill buffs, and any other forms of damage mitigation will not prevent culling strike as long as it meets the life threshold.
Since culling strike is a property of hits, it is not applicable to damage over time.