Disclaimer: it needs to be within a Vault of Tartaros. As such, killing enemies via traps outside, such as lasers, will not count.
There are often Arena Vaults that can be found throughout the islands. These are required for the Vault of Tartaros Heroic Tasks either way, so you will come across them sooner or later. They are distinguished as having an arena to fight upon with several traps scattered throughout the arena floor. Many may not be activated, depending on the Vault you enter, but as long as you do not collect all of the treasures (i.e. the Molten Fragment chest and the Bolt) from the vault, you can re-enter it after you finish it to continue on.
Not tested: I'm not sure if you can strike the enemies down to low health and then have the trap finish them off. The wording of the trophy seems like it could, but I have not done so myself.