It can be done on either echo pack 1 or 2 but I suggests you to do second level of echo pack 1 on casual. Any difficulty don't matter. It's ok to get infected but it will void an achievement only if you are downed by playing online co-op or dead. If AI died, it will not affect the achievement because it only says human player. Hope this help.
In my experience, do solo is more easier than co-op because my friend and I had to retries 3 times because he got downed in first 3 attempts. Hope this help.
I'll make this clear. It's ok to get infected by zombie as long as you have anti viral spray with you but don't become a zombie at all. If you die during the mission, restart the level, not from the checkpoint.
Last edited by Motorexskygtr32; 10-07-2012 at 04:24 PM.