"Armageddon mode" is simply the name for New Game Plus
The good news is that this NG+ can be played on any difficulty. To start, load up the file you wish to use and beat the final boss again (make sure you change the difficulty before you beat the boss as this will determine what you play NG+ on).
Now enjoy being over powered for the run. The difficulty here will be getting to level 100. While I will go into more details in its own guide, here are things to note:
-If you got the "BFA" achievement you should already be around level 60.
-You should already have saved all the humans in the previous run, giving you Riders Mercy which increases lurcher drop rate by 10%
-Make sure you complete the crucible and get the armor that increases lurcher drops by 10%
-While in NG+, enemies with red glowing eyes will now drop items used to upgrade your armor. Upgrade the armor you won from the crucible. This will net you an achievement as well.
-Make sure you have the Balanced Fortune enhancement fully upgraded and attached to your whip (or which ever weapon you use most). This gives you an additional 50% lurcher drops.
-In NG+ you will find "Wisdom shards." These boost the lurcher drop rate by another 100% for a limited time.
-Certain unique drops are now replaced by "Forsaken" Lurcher crystals. These give a large amount of lurchers.
-Save your luminous visages for some of the higher levels to reduce grinding. (You get 15 per run. Its almost recommended just to start a new run and pick these up to cut down on grinding so many levels)
-Going into NG+ 2 grants you armor that increases lurchers while shattering crystals. This can be critical to leveling up providing you save a bunch of them. Also the red eyed enemies drop greater rewards.