Good stuff. This is the first time I'm playing a game like this, so I don't know how this compares to other titles like this (autobattlers, you call them?) but as I play it I'm definitely getting a Civ-style "just one more turn" kind of vibe. There actually is a decent amount of planning and strategizing for such a simple-looking game as you have to make sure you have a right distribution of monsters to optimize current strength vs. elemental balance vs. collecting monsters to evolve them.
Minor note: The chart of elemental attacks vs. defense took a bit for me to figure out (I kept getting what each row meant mixed up), maybe a tooltip that shows when you put the cursor over the cells might help, but then again that might just be because I'm not used to this genre.
I can see myself spending 20+ hours on a polished version of this. Nice work.