KILL 20 enemies with traps.
It is not enough to cause damage to an enemy, the trap must be the fatal blow your foe.
The good part about traps is the radius effect. If you get a few enemies bunched together a well placed trap can really thin the ranks.
Get the Feat, Gunner, to increase trap damage by 50% to really help decimate your foes. You can also add damage by crafting certain armor upgrades, just keep an eye out for them when cycling through your possibilities when looking to upgrade equipment.
You should easily get this in one playthrough unless you forget about them or purposely ignore using them.
You can always make more if you have the components on hand or buy some from vendors like the blacksmiths.