This article's content is accurate to the most recent public game version, 0.4.8d.
"Isabelle uses her heels to kick an enemy right where it hurts. Deals Sharp damage."
Cost: 8 TP
Source: Ganguro Isabelle - Starting skill.
Heellish Pain is a skill that deals two hits of strong 
Sharp damage to a single enemy target.
Given this skill's low TP cost and impressive damage output, this move is worth using against any foe that isn't sharp-resistant.