If you really want to make it clear to players that the situation is hopeless: Totally outclass them with something that obviously isn't even the enemy's most powerful attack.
This is justifiable from a game balance perspective in that you don't want to wipe out the PCs before giving them fair warning, and from a reality perspective in that of course a villain isn't going to use the most powerful attack from the beginning, because which villain would be so sensible as to do that?
The movement of the Rebel fighters around the Peace Moon in response to this order from R2 and Ackbar, by the way, triggered a discussion on the relevance of the hairy ball theorem to to flow of ships relative to the moon's surface. So much so that we almost considered titling this strip "Hairy Moon Theorem".
Palpatine: Why didn't they fire the weapon?
Luke: That was the targeting beam??
Lando: Hola crapola! The Peace Moon weapon is operational!
Admiral Ackbar: Fortunate that we're here to contain it, then.
Lando: We need to get out of here and regroup!
R2-D2: This is good! The Lost Orb is there! And look, the Moon isn't that manoeuvrable. Just stay out of the firing arc!
[SFX]: < fi-bippity bibizung fodooby bop bebedooby-bing boo-bippity-ting ziting bebioo poppy-bizung-zang >
Nien Nunb: I die honourably for my love! Remember me fondly, Mon Mothma!
Admiral Ackbar: Fleet, flank the Peace Moon!
Han: Right! Go from an attack on all fronts to an attack on most fronts.
Lando: Which fronts?
Han: All the fronts except the frontal front!