The thing is, a creature taking the physical form of a human(oid) would have trouble hanging on to anything in the gale-force winds we see when a bay door indeed is opened in "Disaster". So this tactical maneuver might achieve something, after all: the creature would be blown into space, perhaps outside the shield perimeter, at which point Picard could slam on the shields and hopefully defend the ship against the creature that way.
Then again, the other thing is, such gale-force winds are not very realistic. If a big room like that suddenly loses one entire wall to vacuum, the air gets out in a rush, yes - but it's quickly gone, probably faster than it would take to shake a human standing in the middle. And the closer the person stands to the back wall, the less effect there will be. Unless, of course, our heroes in "Disaster" forgot to shut down and lock up the ventilation, and it wasn't just the cargo bay emptying, it was the entire ship... (Sort of defeats the stated purpose of depriving a threatening "plasma fire" of oxygen!)
A pressure change or a phaser blast or a hail of bullets through the body or a lightning bolt might all affect a godlike supercreature if it were taking the form of a humanoid and was somehow dependent on this form staying intact. At the very least, there would be some disorienting surprise. But I'd still expect a creature capable of "becoming human" to also be capable of "becoming itself" again, even if said human were reduced to a cloud of torn flesh and boiled and frozen blood by an extreme pressure change. (And in order to be that extreme, the depressurization would have to begin from something like a hundred atmospheres, not one!)
Timo Saloniemi