As far as I can see it should seemingly activate right after the turn you damage either of them. So you would have to beat Veronica in one turn to miss it.
Over on the page for the Veronica boss battle the dev mentions that should be a bug. Either it wasn't fixed or the current setup where you only need to do any damage was considered the fix and they just didn't expect you to one-turn her. @JackyHF?
Also yes, it was an extremely useful spell =P