New information has caused me to make another new plan.
[X] Plan Kill the Darkness
-[X] Start by casting Guardian Touch on Arc Mastermind, Shifting so that when you cast Wave Force you won't dispel Guardian Touch, hitting the shadow with Wave Force, telling Arc Mastermind that you'll be back soon so please try not to die, then Shifting away to the nearest fight.
-[X] Purposefully taunt the demons into attack you so that Ceaseless Warrior is powered up, and try to get as many of them to attack as possible.
-[X] Once Ceaseless Warrior has been powered up to a decent degree or two turns pass (whichever come first), Shift back to Massy and change your Panoply's configuration to Multi Shot, Danmaku, Horrorslayer, and Heavy Caliber. Destroy as many of Massy's legs as you can in one turn, and if you have enough shots left over, try to get rid of its arms as well.
-[X] Try to use Wave Force each turn in such a way that it disables Massy's spell casting ability, while leaving Arc Mastermind's intact. If needed, use teleportation to achieve this.
-[X] Once you've removed some of Massy's limbs, switch your Panoply's configuration so that you have Crush, Intercept, Double Hit, and Adamantine and help out Arc Mastermind by engaging Lunatic in melee. Let Arc know you can survive against Lunatic before you engage the latter
-[X] Make sure to dodge and negate Lunatic's attacks when needed, and use your teleportation if it's needed.
-[X] Ideally, try to stall until Team GEAR gets here so they can finish off Massy
[X] EXP Plan Kill
-[X] Purchase Horrorslayer (420)