Irresistible Blow

Acquisition
Skill Quests: Skill Trainers:
 * Prophecies
 * Securing the Vale (Yak's Bend)
 * Prophecies
 * Firstwatch Sergio (Lion's Arch)
 * Dakk (Ember Light Camp)
 * Factions
 * Masaharu (Seitung Harbor)
 * Nightfall
 * Dau (Mouth of Torment)

Trivia
The creature appearing on the skill icon looks very much like Kanaxai, the boss of the elite mission The Deep.