so you tried reich runners? post your current record on it. lets see if your any good. the next strategy is my personnol creation, called trapezoid.
starting out, knife, grenade and shoot your way to round 20. you should have a browning or something as good, and a ray gun would be nice. now make sure the only openings are the help door and the stairs in the help room. no more. now fine the corner where all thats around you is a blown up door frame and a window (behind you) this works great with a buddy, having one guard the window and one guard the door. just do this untill it gets to hard (around round 30) and then use reich runners until round 40. then get even better weapons (flamethrower) and hide in this corner. use this same every ten round switch strategy (if your unexperienced switch every five or two rounds.) do this and youll probably get to round 70-80.