I really enjoyed this! This type of progression just ticks for me.
The game was really hard and left very little room for error in the first couple levels, forcing me to just run around the arena waiting for the mana to refill, so I believe it could use some balancing, especially as a way to allow the enemy spawning to get even more out of control without necessarily overwhelming the player.
I would love to see something like different types of spells or whatnot, and a way to recover Mana other than just waiting around.
The pixel art is simplistic but well made and the audio is a bit repetitive but nice, this is a really nice game, great job!