It's one of Ignis' techniques. I don't know about that other guy, but the weapon that seems to pair off the best with it is the Royal Arm, the Bow of the Clever. For whatever reason, the Overwhelm skill just makes you shoot a shit ton of arrows at a pretty fast rate, on top of your teammates attacking, so it's a lot of easy DPS. And if the monster is "vulnernable" when you use it? Good lord...
Correct. At level 70 I was probably dishing out close to 100k each use of the tech. All while invincible lmao.