I will give my phone to technical service due to a problem on battery. I am a little paranoid about technical service employee and don't trust them in terms of personal privacy.
I read that factory reset does not permanently remove everything on phone: http://blog.avast.com/2014/07/09/android-foreniscs-pt-2-how-we-recovered-erased-data/.
I would like to delete everything on phone in a way that in which even high level forensic recovery softwares can't recover anything.
By the way, i am not an expert user on android so please describe the solution in detailed form.