This is a very creative use of RNG in games. I like the mechanics of the game, it makes for a fast paced game as you try to wrangle with the spells.
Some critical points I can give:
1) The health potion spell seems to buff the monsters? I don't know what some spells will do. Some sort of description of spells may be beneficial.
2) In my opinion, I think the mana system is not necessary. As you may run into a situation where you cannot cast any spells due to insufficient mana, causing you to be a sitting duck. This will also incentivize sitting in a corner, waiting for the mana to recharge.
All in all, this is a interesting game with "tetris-like" randomness mixed with top-down spell shooter. With more polish, it has potential.