I am an atheist who has read the Bible and written several books on the topic. Now if you want a 20 second soundbite to win any debate, okay here goes. Start with the OT story of creation. Either the snake talked or it didn't. Either the earth is 6,000 years old or it isn't. Pretty simple. If they claim the snake talked and a young earth, you should be able to laugh them out of the room. If they claim it is just a parable, then use the fundie debate. If there was no real fall of man, if it was just a parable, then why does man need to be redeemed? There is no need for Jesus. It is a lose-lose situation for any Christian.
"On Earth as it is in Heaven, the Cosmic Roots of the Bible" available on the Amazon.