It's a pretty mean angle to take, but why deconvert people by pushing them into a nihilistic crisis?
It's not like atheists think life is meaningless, kindness to be pointless, or the afterlife something to be anxious about.
I've found far less mean-spirited success by explaining how belief isn't necessary for existence to be worthwhile for us. If they can come to understand how happiness is possible for someone who doesn't believe, their own belief suddenly become a lot more optional.