Item description:

One Down - Play the mission. Best strategy is to hide in a bush and use Attractors/Attractor bombs, then grenades, pipe bombs and molotovs to kill them. Apparently you can also die too much and the game will let you skip the mission but still award the trophy, but I tested this by purposely dying 10 times on Normal, and I never got a prompt to skip it. So either I didn't die enough, they patched this out, or it only applies on Easy.