The main causes of chewing hex heads is a poor fit with the tool, not all hex keys are created equal, I have a set of Wera hex plus and they are really good for those tight screws.
https://www.youtube.com/watch?v=zi-i1bRWzFI
https://www.youtube.com/watch?v=eRZjNYGHeRQ
But if you have something that is rusted up and seized solid then you'll need to go for the more brutal methods of penetrating fluids, shocking, and heat.