There's been a lot of great analysis on how good this final shot is but I'd like to bring up a new point.
Namely how hilarious it is in light of how the Ironwood defense squad used to justify his actions and try to paint RWBY's suggestion of calling for help to fend of Salem as stupid by insisting that only Atlas has a military and that Vacuo in particular would be completely unable to contribute against Salem's invasion of Atlas (pay no mind to Tyrian explicitly stating that "all would be lost" for Salem and her cabal if Ironwood called for aid from Vacuo).
Because man look at this fucking shit. There's clearly meant to be airships from all 4 Kingdoms there and we know which to type belongs to Vale, Atlas and Mistral. Which means the new airship type with the giant fucking cannons belong to Vacuo.
But oh no, I'm sure having a fleet or three or those ships showing up alongside hundreds of Huntsmen, a most likes fully trained and loyal Summer Maiden* and whatever form of ground forces Vacuo has would not be useful against Salem at all.
We all saw how powerful the dinky little "heavy cannon" on a cruise ship could he so I'd imagine those spinal cannons are pretty damned powerful.
Atlas wasn't that far from being able to nuke Monstra on their own anyway. If Vacuo had shown up with a fleet and helped blast Tempest Grimm out of the sky to give the air forces more breathing room then that alone could have turned the tide of the battle enough that Oscar might not need the cane. Not even mentioned how big a game changer it would be if Vacuo does have a Maiden in her prime at their disposal.
* Because at this point we've already played through every single variant of Maidens being indisposed for the good guys and reached a point in the story where a Maiden wouldn't overshadow everything so it's high time that we see a Maiden that just works.