It is possible to defeat these enemies without ammo. I somehow managed to finish this level without using any. After reviewing the map in Pfhorte though, I recall that it was definitely not easy.
: Unlike with our last journey to the GenCorp ship, we can pretty much return
: to our little safe outpost any time we wish - and we'll need to! Things
: have definitely been ratcheted up a notch and we now face some new
: monsters to make life a bit more difficult. In addition to the green and
: white GenCorp Drones, we now face cloaked drones that shoot white energy
: bolts. Even more difficult are the tanks - real tanks driven by GenCorp
: soldiers - they fire bullets and grenades and are oh so hard to take out.
: We'll waste a lot of ammo dealing with these babies!