Pretty easy to get here. Just go to administration area of the prison and find an area that is swamped in the green joker gas. Dive into it and wait. You have to wait for the green meter to fill and then after that meter is full, Batman's health will slowly start to deplete. It takes awhile for all of his health to go down but once Batman dies, the achievement will pop. Good luck and if anything is missing here, let me know. Thanks!