The best way to do this is to use the Spartan with the Xyele and Bronze Curiass (and ensure difficulty is set to normal). All you pretty much do is run directly at your enemies use a simple , , combo before they can get an attack in of their own, if all of the combo hits it should kill them outright. Just be aware of the Centurion when he has his medium range weapon equipped, be ready to block. The same goes for when a freshly spawned enemy pulls out a ranged weapon, quickly block and run in. I managed to get 44 kills using this technique so it shouldn't be too hard.