How does a pure rogue thief acrobat utilize no mercy. As far as I can tell, there are no rogue abilities that induce "Helplessness" on an enemy. There's trip, but that's just a knockdown effect. Assassin has assassinate, Mechanic has wrack construct, but I don't see anything that would place an enemy under the helpless condition.
Stunning blow perhaps, but because the Acrobat's trip abilities, damage, and attack are dexterity based it makes it hard to want to take that.
Is no mercy meant for cross classes?