Just be good. This one sucks.
Best game mode to get this done is probably "Last Titan Standing" as you don't have to wait until you "earn" your titan and there's always one available to shoot at. ;-) Once you get the "Capacitor" attachment, use it - that makes the gun a "Titan Charge Rifle". Try to keep the weapon charged half way or so, so you'll be able to aim and shoot within half a second at the next best titan behind the next best corner... Couldn't seem to get it together in "Hardpoint" within hours but finished ~20 Titans within fourty minutes in "Last Titan Standing"