FWIW,
I found several zoanthid colonies in my tank that had nudibranchs on them.
I setup two different dipping bowls. One with ReVive and the other with FWE at 4x the recommended dose.
I watch the nudi's pretty much explode in 10 to 15 seconds in the FWE bowl and after 2 minutes the nudi's were still moving in the ReVive bowl. I didn't wait to see how long it took, I moved that colony over to the FWE bowl and they died within seconds.
I also used FWE to treat the whole tank, twice.