Wow. I just got the Left 4 Dead 2 achievement last nite while checking out the new Mutation "Last Man on Earth". As long as you use an adrenaline shot as soon as you hear the Tank, you can easily make it to the helicopter in three minutes without having to deal any pesky zombies. Freakin Awesome.
I know it's rather late in the week, but I just thought I'd let yall know before they replaced the mutation.