Just wanted to clarify and confirm that you DO NOT HAVE TO PLAY ON HARDCORE for the 50x multiplier and "Alien" rank acheivements! Hardcore mode was designed for 4 player co op, not single player.
I can't believe I spent hours trying to do these on Hardcore because of this acheivement guide and roadmap, I'm quite angry actually for not getting your facts straight on this. Not only was getting the "Alien" rank and 50x multiplier cheeves much more easier, I got them on my first try! I must have attempted these achievements on hardcore a dozen times a peice to no avail.
I'm just happy I chose to actually read the acheivement descriptions (none of which say that this has to be done on that god forsaken Hardcore mode) and knew it made absolutely no sense when you guys say this needs to be done on hardcore.
People, do these 2 acheivements on Normal, if you are lucky, you might even get them on your first try like me.
Other than this blunder on your part, this is a great guide other than your advice on these 2 acheivements. And yes, the 2nd stage of the DLC is the best way to get your 50x multiplier, just be sure to DO IT ON NORMAL!