This is one of the levels with a glitch where you can get a very fast time by doing (almost) nothing and let your teammates do the work for you.
Restart untill you spawn in the little green room with the health pack and the plasma rifle in it. Grab the plasmarifle and just crouch there facing the spawnplace. Your teammates will kill for you (very) fast. If an enemy respawns in the room, stick a grenade on his back. Watch the radar for enemys that come up the stairs to you. If one appears in the door opening also stick a grenade on his back. But then try to stay away far enough from him, or you'll be blown up with him. Thats the whole strat. Using this strat end times can vary from 1 minute to a new w.r. But if you're persitent a time around 45 seconds is easily achievable. My son of 10 ried his luck once and had a time of 44.0 seconds on his very first try.
Questrobin.