Are you talking about the Instructor fight? Because outside of that nothing in Naxx 25 needs to be MC'd or CC'd. In the instructor fight you've got plenty of time to gain aggro. Without them needing to MC the Understudy you're tanking. So once you get enough aggro then just stop the DoTs. I use that fight to level my gun skill while the Understudy smacks me. Not much to it.