When you see a Hwacha pointing at you, automatically assume it is gonna send a volley your way. Keep your eye on it and as soon as you see the glow of a rocket, use an evasion tool like Chute Vent, Hydrogen, or Moonshine. If they're far enough away, only a few rockets will hit you and now they have to reload.
I favor Chute Vent over the others as it has the fastest position displacement, plus, they'll probably hit your balloon, which doesn't hurt very much.
If they're close, you should already be moving. It's best to always figure they are about to fire at any second. And if you do get hit, try and get at least one gun on them while the others repair, otherwise all they have to do is wait for their gun to reload. Get a flamer or Carronade on them if you can, or a Gatling preferably.