Ride 'Em Cowboy To jump on a titan (Rodeo them) you have to get close to them and jump on them with the A button. Once you are on them you can shoot the hell out of the wiring and it will go into doom mode, you will unlock the achievement after the match has finished.
Note: Sidewinder anti-titan weapon will kill you (or any anti-titan weapon) if you try to use that while shooting the wires.
According to peoples responses, it must be a manned titan, and they cannot eject, if they eject you will not get the achievement.
Have fun and Good luck!
Video: A great job shown by EliteSnipersRF!