This article's content is accurate to the most recent public game version, 0.4.8d.
Cost: 9 MP
Source: None
Sap Strength is a spell that deals moderate 
Absorb damage and applies ATK Down for four turns to all enemies, also healing the user for the damage done.
Any enemy that has this spell should probably be focused on, as draining HP from a party-wide hitting attack can give them a lot of sustainability.
The following enemies can cast Sap Strength: