Indeed, using the environment is very important. I die almost every time when i try to fight the gankers at open areas. Except when there are some npc's or other enemies on my side. It's easier to avoid getting backstabbed if you have some trees and stuff around you
I have recently come up against combo's that involve a melee person and the other using the moonlight greatsword. I can't tell you how much having the trees in the area have helped me, but I use an ultra greatsword so they also hurt me a bit too.