Is just to easy to avoid it. I finished the game in legendary and never saw a Scylla using it and damage a single soldier. Player already has a huge advantage over pandora in late game, so let the Scyllas fire without warning… this will increase the difficulty of late game (what is severely needed imo).
Canny Official Feedback Tool topic:
https://feedback.phoenixpoint.info/feedback/p/remove-preparing-fire-for-scyllas-blaster-cannon