Kill It With Water: Leaving aside such not-really-the-water examples as water knives, pressure effects, and drowning, this is rather rare, inasmuch as the universe is full of the bloody stuff and as such having much of a vulnerability to water is unlikely to let you live long enough for anyone to actually try to kill you. (And any species having one would be unlikely to be so freakin’ stupid as to invade a small sylithotectonic world whose surface is something like three-quarters covered in the stuff. Yes, I’m looking at you.)
Perhaps the best known related, not-really-the-water-but-more-so-than-some, example exists mostly because of those species whose native temperature range is such that water’s natural state, as ice, is a kind of rock. And being hit by a stream of liquid water, therefore, is much like immersion in lava.