The protection on the enemy soldiers, which the fleshers have wrapped around them despite.
The enemy soldiers inside The Ball of Flesh carry an invulnerability effect, but the Fleshers have mostly wrapped themselves around them anyway.
Rosetta warns that casting Laundry Day might remove the fleshers and negate the effect, causing the fleshers to attack their own soldiers instead — an unacceptable risk with Eris and Meatus in the mix. The plan instead uses War Crime to combine Bijanbi and Wall of Fire, keeping the fleshers attached so they can serve as the finisher.
The enemy protection whose interaction with the fleshers shapes the entire plan.