For this achievement, you have to run up to any bad guy and press . It does not have to kill him; you simply have to just land the melee attack. It can be quite hard to do on the harder difficulties. If you don't get this achievement during your normal playthroughs, try to lower the difficulty and play Chapter 2, running up to enemies and using the melee attack.
There are multiple places to boost these kills. It can be boosted right at the start of Chapter 2, even on higher difficulties. Near the beginning, run down the ramp and 2 guys will be standing near the edge of the walkway. They won't turn around, so run up to them, press to kill both of them. Now look right, there will be a 3rd enemy, so quickly run down to him and press again. Now simply restart the checkpoint and repeat. It is also easy to boost right at the beginning of Chapter 6. Also, remember that an execution does not count towards this achievement. They must be standing and alive.