First, I'd suggest waiting for this until you're near the end of the game. Ideally, any time after you've met with Father and have returned from the northern reaches of Hope County will allow you to acquire the Bliss perks, which would be a good heads up point. It'd also be easier to just do this from a Rank 3 outpost, since many Enforcers start to appear at this point (at least 2, more if you allow them to call for reinforcements).
Whittle away on the Enforcer's health until they're around their final bar of health, then run up and melee hit them with the weapon equipped. If done successfully, you'll get an animated sequence where you kill them a bit more stylishly, which is the condition needed for this trophy. If you do not lose control of your character during the execution of the melee attack, it didn't count and you'll need to find another Enforcer or try again if it didn't kill them initially.