Quote Originally Posted by whoopeeragon View Post
What happens is you need to do enough damage to it to interrupt. On one of the last ones, however, another cactuar pops up and uses Cover on it, so you need to kill that add as fast as possible, then change to the boss.

Since you said you burned everything down, you probably burnt the add and then the boss and didn't realise what the exact mechanic was. xD
Yeah, that's what most likely happened. Flawed mechanic!