Titan Killer (XO-16)

is one of the many challenges in Titanfall.

How to Get
This is easy to do if you team up with a small team of people, but it's also possible if you corner a Titan. Just round the Titan and attack from behind, and you should get a load of hits in. Keep in mind it's only the last shot which kills that counts, so you can easily dispatch the Titan with rockets, and switch to this weapon for the last kill.