Don't bother trying to melee enemies (especially if you are playing on Insanity, like I did from the start). You can easily get it if you find one of the Space Monkey colonies - there is one on Eletania in Hercules system, on south-eastern part of the map. Go there, exit Mako and save your game. Melee kill 10 monkeys (they die from one hit), reload and repeat. Kills are cumulative and carry over if you save and load. Reload 10 times and voila!