This achievement relies on the Many Must Fall capstone skill from Zer0's Bloodshed skill tree. While in Decepti0n, this skill deploys a new decoy and adds two seconds to the countdown if Zer0 kills an enemy with one hit using melee. This can be repeated an unlimited amount of times.
An easy way to complete this achievement is to seek out a low-level bandit spawn area in an area without other disturbances, such as the bandit camp just outside of the entrance to Sanctuary. Once the bandits have spawned it is only necessary to pick out the weak ones and kill them one by one before the timer runs out.
Some helpful skills for this achievement from the Bloodshed tree are: Killing Bl0w, Ir0n Hand, F0ll0wthr0ugh, Execute, Backstab, and Like The Wind. Strength relics, Maylay shields and weapons with melee attachments are also recommended for enhancing melee effectiveness.