This is a fun game! It's simple enough yet takes some time to master. The game is extremely addictive too. Played a few rounds because of that. Quite like the synergy of different elements, that should spicy up the game play and adds a bunch of replayabilities too. Nice job!
For some nitpicks, I feel like the game is getting a bit stale after reaching certain levels, there is less choice for upgrade during late game where you are no longer choosing strategically. Maybe because there aren't much to change at that point in the game? I am not too sure. Also, although the elemental attack system is nice, I seldom use that during late game, where it doesn't even matter what you are using you are still killing enemies with the same speed, and the split of attention changing attack type often bring more harm than good. Such that in all of my playthroughs, the whole elemental system are just not being used.
But other than that, the game was so nicely built, I'd say its a job well done!