Drogue Chute (I think that's how you spell it?) helps quite a lot, minimizes the speed at which you fall.
Pyramidions are also a pretty good choice to take against them as quite often the carro hits will also hit the hull covering the balloon, forcing 3-4 shots from the carro to be shot before the balloon is taken down; given how long the reload time is you can often kill a fish during that time.
Keeping yourself at long range is often an option as well, artemis/hades/light flak is good. Sticking a merc somewhere can also solve your problems, provided you hit the right component with it.
Heavy Carronades also do poorly in vertical arcs, so being above the level of a blenderfish will do wonders.
That having been said, yeah, on Paritan Blenderfish are ruthless. They're also pretty good on Duel at Dawn, ok on Canyons depending on where the fight takes place, but don't really do as well on Fjords or Dunes.