Was this that Mel Gibson / M. Night Shyamalan movie?Unless they are allergic to water. Saw that in a movie once.
Yeah, I was thinking the same thing. Aliens might have more advanced weaponry but their physiology might not allow them to roam the Earth without some type of protective barrier that isolates them from the atmosphere.
They might be able to breathe our air and water might actually be harmful to them.
My thought is that they would just wipe us out from the comfort of their starship. They wouldn't even have to come down to the surface to attack us.
However, if they were to want to come down to the surface, I'm sure they would be able to figure out if our atmosphere and conditions would be suitable. If not, they could put on some sort of suit to protect themselves. There's no way we can beat any species that can travel from another star system. Data famously makes this point in S03E02 of ST-TNG.