Hey guys I'm just in sharing something I found out. If you go to the fourth mission where you have the beacons you can get into part of devils lair. You just keep running until you come to an area where you go into a sort of rock tunnel. Then there will be guys you cannot kill, you run past them. Then you run past more guys including a knight and shriker. That takes you into a gave with more invincible guys which you run past. Then you are in the refinery and have all level 8 guys to fight. You kill them and go outside and you can fight the devil walker.