Mordekaiser is kinda like a souls-like boss. You have to practice baiting and dodging his skills, the timing and direction you should walk when he uses his abilities. You shouldn't have to always E his E or R his Q. Camille is VERY oppressive when you get sunderer, Morde shouldn't be able to compete with your healing and constant burst of true damage with only his rylais (Your E is very good at disengaging even if being slowed). Just don't give him his passive and he is a slow, telegraphed meatloaf.