Dismantle was a debuff thrown at a single ennemy:
10% reduced damage, 5 second duration, 60 second cooldown.

If this sounds very strong, it's because it was very strong. Its main weakness was that you needed a target and was only partly efficient against multiple targets.
But those weakness paled in comparison to its strengths and flexibility.
In today's application, you would simply move a party mitigation to somewhere else if you can't target the ennemy.