If I can do it solo on Master in under 20 minutes, I am positive anyone can do it on normal in under 40 minutes, it just takes practice and a good loadout doesn't hurt. I can't tell you how many times I failed to do the original Whisper mission solo before I finally got it done not long prior to sunsetting, and that was "normal" and 20 minutes. I'm not special, even this updated Zero Hour took me dozens of tries to solo the Master version, it honestly comes down to repetition and having solid damage with survivability. You can get it done if you really want to, believe in yourself!