Questrobin - Disco Inferno

Immediately go downstairs to the dead end (after room with the big pillar). grab the tommygunx2. Crouch in the small passageway which connects dead end and the room with pillar. In the beginning the other characters are not infected.

When one comes down to you let him pass so he can get in the dead end, BUT BLOCK HIS WAY OUT! With some luck all three not infected characters are behind you and cannot be infected (I had two behind me. On the two or three times the infected guy comes downstairs, you can kill him easily from the distance with your tommygunx2.

You don't even need to get new ammo with only one (or two) infected guys. This way lasting three minutes is a peace of cake! This strat worked fine for me.