i used the smartgun to get through the whole wave of aliens and then the 2 big mamas. If you use it accuractly rather than just blasting and wasting ammo, you can finish them all off and still have about 100 or so ammo to spare.
I found that walking up to the praetorians (or whatever they're called) to a certain point will only make one of them run at you. I just ran backwards and reloaded when one was dead, then attacked the other.
Online Gaming Quest Priorities:
Alien Breed Impact 3, Arcana Heart 3, Army of Two: 40th Day, Blazblue, Brutal Legend, Call of Juarez: the Cartel, F1 2011, GTA 4, Hard Corp: Uprising, Inferno Pool, Operation Flashpoint: Red River (+DLC), Quantum of Solace, Street Fighter 4, White Knight Chronicles