Cactace is really good but she can't do the whole hard stuff by herself, even if she can attack twice. That's why other single target nuckets exists and can kill a Nidhogg in one turn before it rape you.
And you can't really take Cactace as ultimate and unique 1 target attacker when the game has a monster called Kerria (and another named Rainbow Rose but I can't speak for her).
But I'm totally okay with you for Purple Columbine. She is not bad but not good. She is just good to help your boss nuker team when you don't have anything better.