@TheWindelHere is the key phrase that tells you what the ability does: "
Upon inflicting what would otherwise be damage* upon a being, Artoria may instead transform them into a cute animal girl in proportion to the damage they would otherwise do."
This is a phrase that gives an
example of what the ability does: "Ghosts, pure energy beings, and immaterial concepts can all be punched into cute animal girl-ness by Artoria's reality warping obsession."
This is like saying "I have an infinite storehouse of weapons. I can pull a rocket launcher out and shoot someone with it." That does not mean the infinite storehouse of weapons is limited to rocket launchers.
The very first time I used this ability, it was a ranged attack using ice. Nobody complained about the ability being ranged.
@Lmpkio Unless I made a grammatical error, everything was phrased in the "Unless Mephilius dodges, this is what will happen" sense. So, you still have like, a minute or two before the lava finishes flowing ALL the way down the tower and doing that. Artoria's at the top, you're at the bottom, and although this is very liquid lava with all the extra heat, it still takes awhile for gravity to do its magic.
That said, if you do get hit by the lava, you'll be able to temporarily access its stored power. Artoria supercharged the lava with both your pyrokinesis and Ira's black flame. As you're a fire manipulator (the one who gave Artoria fire manipulation, in fact), you'll be able to access that charged fire with your own pyrokinesis, getting effectively a free charge for your attack, like Eos did for Grafdakka, and also temporarily getting the Black Flame as a nice Buy One Get One Free to surpass your own limits. Of course, when you run out of lava, you'll be back to your own level of power.