Sadly you can't unless they want to get better because MMO's have become a community that just can't really deal with "criticism" because "muh feelings". The worst part is that the way people approach towards "getting better" revolves around "numbers" and not actual input into understanding "skill".

Personally I see all revenues being unproductive. The only way to go about it is having an actual 2 version training guildhest where in 1 your are forced into your personal rotation and situational awareness. Then the second being how to react as a group and using the other abilities in your arsenal that you wouldn't really touch in 1v1.