u can use strong language but not a defamatory one.
u bring in Quran’s example, but are forgetting that u r not The Quran 
Prophet Muhammad PBUH didnt use abusive language for anyone…otherwise we are human, and not perfect at all…but still ALLAH SWT send his blessings on us, and many kafirs and non-muslims. 
so try not to do what you are not made for.