How many epidemic cards to use with the Bio-Terrorist challenge?
The strategy in the Bio-Terrorist variant is sufficiently different that it probably makes sense to start at 4 epidemics until both sides are comfortable with how it works.  It is easy for players to be too focused on stopping the terrorist and lose to the normal mechanics, or to completely ignore the terrorist and lose to the new mechanics.  A balance between the two is required.

Also, make sure that you are following all the rules for the Bio-Terrorist's actions.  Specifically, many of his actions are limited to only once per turn.