The Last Remnant

Cachexia is a Hex spell that is done by Hexes specialists. If you don't have a Hexer started early in the game you probably won't have time now to get Cachexia as it is an advanced spell and takes a long time to get.

Instead, use 3 unions. Put your heavy hitters in the first union, and make sure Blocter is in that union. If you haven't been using him, take him out and do a couple of battles first so that he asks you for the Wonder Bangle. That Bangle will heal up status effects at the end of the round, so you want him in your hitting union so they can keep hitting. Use two other unions for healing, keep one of these on Standby at all times. Do everything you can to keep the hitting team healed so they can hit every round if possible.

Do as much damage as you can in the first round with all three teams, then back off and just hit with one or two teams.

You can tell you are ready for the battle if your teams can hit 40k damage on regular monsters. Once they hit that much, they should be able to take down the Fallen in 10 turns or less.