Only thing I can think of atm is maybe you aren't 'aggressive' enough. Use aoe smartly, and try to kill as fast as you can. You have to kill them before they kill you. Always keep moving, but not only backwards.
[quote name='shanaeri rynale' timestamp='1373291374' post='2217638']
Are the chests you get RNG? As this part Seems to imply they are..
In the pictures where it shows a large achievement chest, there's text that says a choice of: and then it lists several weapons, so I don't think it's gonna be rng.