Water

NOTE: I have decided any alternate chapter to Earth is subsequential. So

I punched a common villain who calls himself The Dragon with my rocky fist. He can breathe fire, is apparently immune to flames, and he threatens to set this two-story building ablaze with this ability. With everyone else having been evacuated the building, it was only me and him, no innocent bystanders. Every time he blew fire at the carpet or wooden wall, I had to stomp it out with my stone body, which is pretty slow to move in but at least it was impervious to the flames, switching back and forth from my regular (and faster) self to my rocky self would have taken even more time because the process is pretty slow for a combative move, it takes a minute, and there's always a risk that I could be burned in normal form. Even with my gray, tight, fireproof hero suit and mask, I would still feel the heat in my regular form, not to mention my face and neck is still exposed.

"You're slow, Rock Hard." The Dragon said with a grin, it's the name the city had decided to call me for reason I don't know, but I wasn't creative enough to think of anything else, so Rock Hard it is. "Bet you can't stop me." he took another deep breath that indicated he was going to blow another huge shot of flame. I needed speed since he's at a distance. So I took a minute to have my stone body turn to flesh, starting from my arms, then inwards. I then ran to him with my long legs as he's already blowing up the wall. I've always been a faster runner thanks to my long legs that give me the height of 6'8.

As I ran towards him, the ceiling fire sprinklers went off, spraying water in attempt to extinguish the huge flame, getting me and the guy wet in the process. I pounced him onto the floor and pinned him, face to face. He inhaled another deep breath; I could see his throat glowing a little as I could see what was about to happen. I began to turn again, but it was too late. He blew at me, and I felt really hot, like I was boiling. I let out a loud scream.

Luckily, I was saved by the fire sprinklers. Unluckily, I was naked again! It turns out I had another power within me, instead of turning into stone, my body was now made entirely of water, and my size seemed to increase as the shower of sprinkler sprayed more water on me. The suit was fine, but my liquid form had phased through them, so I was a growing mass of water body, and if I turned back, I'd have my body, along with my secret identity revealed to this villain. The Dragon looked at me with a look of surprise as I grow bigger with the collected water. He stood up and ran towards the nearest window. I followed him, turns out 'running' in liquid form is much easier when you're just rolling in a big round shape, no need for legs. I was flowing like a flood. I extinguish flames along the way and kept growing as the sprinklers kept spraying, and my superhero suit was suspended within my watery form as I pursued the villain.