I thought it was indeed a sentry gun/turret you needed to man to destroy the walkers, double checked our walkthrough
and that's the solution. (**Warning: Link Contains Spoilers**)
In any other situation, say, multiplayer, you may not have such a luxury, and will be forced to use other tactics, like lightsaber to the legs (while circle strafing to avoid being stomped and crushed!), rockets or other heavy weapons, td's thrown with care, and even the DEMP gun. Force push can be used to send rockets back at the vehicle, and the saber may be able to block some of its shots.