From what I can tell, smoke just makes the game harder by hiding numbers. But I think it's always a little close to fire.
The scroll is a magic spell and I noticed 3 effects so far. Shows you level 1 monsters, puts out fires and let's you collect XP from them, and shows a couple of random monsters.
Rocks are just a gamble. Sometimes they give 3 XP for 1 or 2 taps, sometimes they give 1 XP for 3 or 4 taps. I like to tap them before hitting level up if I have health to spare and no monster to kill with that health.
This is all from like an hour of playing so it might not be super accurate
