Skeletal Avenger

So the bad news is that the exploding barrels do not seem to count for this. However, you can use perks that increase environmental damage and get low level enemies to follow you around spikes and poison for damage. The achievement is cumulative and you can see how many kills you have in the stats section called "Trap Kills" by clicking on the green mirror in the main area before starting a quest.

If you take some time to kite a few enemies through spikes on each quest with damage perks up, you should acquire this well before hitting the 50 quests completed achievement.