How to Beat Pandemic on Heroic Mode (6 Epidemic Cards) without ever leaving Atlanta:
You may not win all the time, but you will win. This was tested in the App as well as on the PC version from Z man games.
To do this, you must play with two players and must have these roles:
A third player can only be added if you decide to buy the "In the Lab" expansion, where the Virologist is introduced. If you want to use a fourth, you must get the "On the Brink" expansion as well just for the extra event cards, where you can add the Contingency Planner to spam Events. Keep in mind though, you still need to use these roles with the base game rules only.
In order to win, you must use the researcher as a storage tank for the cards, and only give what is needed to the scientist. Basically, the turns just result in sharing knowledge, curing or passing. You may also treat disease if cubes appear in Atlanta, but that is about it. You do not move from Atlanta for any reason and you use your event cards whenever needed (or discard them to make room for city cards).
What is funny is that you can actually beat the game very quickly doing this method, sometimes much faster than the conventional way. In one game we played, we ended up curing all 4 diseases within just 7 turns, which is the fastest we have ever beaten Pandemic on heroic mode.