Towards the end of the Ausmerzer mission (where you have to hijack the flying nazi ship), you'll encounter two very large robots (bigger than the supersoldat you've been fighting up to this point). Kill one of them (I'd recommend the Kampfpistol with the rocket mod), then clear out all the infantry, otherwise they'll swarm you (and you really can't defend yourself while holding the Übergewehr). There is an area underneath the big arena that the robot can't reach, so you can funnel enemies in to mop them up. Finally, pick up the defeated robot's gun and kill the second robot with it. It's basically a black hole gun that you have to charge up with
in order to fire. It'll take a few shots (and you'll have to hide between each shot), but it'll go down easily enough.