I personally was active the entire time; however, I heard some people say you can stand still? Regardless, to be safe, I recommend that you be active the entire time. It seems that there are instances where the achievement won't unlock if you stay still, so just jump around and attack or something lol.
From what I've read, I think it will be, seeing as you have the option to customize pretty much everything. And I'm not positive, but I think it will be possible to earn achievements even if you customize the game. So, hopefully it'll be possible to give yourself crazy high weapon damage and make the enemies really weak. I guess we'll have to wait til it comes out to find out though.