Brass Knuckles (Melee Kills)

is one of the many challenges in Titanfall.

How to Get
Killing 5 Pilots as a Titan is relatively easy, but with melee, it's a tad harder. The easiest way to do this is to rush into a group of units and use your melee attack. You'll hit them about half the time (the other half crushing), but if you're effective in movement, soon enough you'll unlock the challenge.