Yeah, it's fun and I've beat it a few times but agreed that the balance is a little off. You have to really rush everything to stay ahead, it's a frantic game to stay ahead of the curve.
Since towers fire slowly (and it seems like the monsters can only take so much damage at once or have invincibility frames?) it's better to spread them out along the first row. Once you have 10 -5 towers you should be able to start buying -10 towers. I usually have 5 -5 towers at the top and then the other -5s spread through the second row, and the -10 towers on the bottom row. Once you're buying -10s there's no reason to buy -5s. Last game I won with 100 health was 11 -5 towers and 7 -10 towers
In all of the games I played, I was only able to afford the ÷2 tower once, and it wasn't helpful as the monsters were already ahead of the damage output. Seems impossible to be able to buy the ÷3 and still do enough damage to the monsters. I think it would be better with a little tuning to make more money, make monsters scale more slowly, space them out a little more, or something.
Really cool idea though to use the math as the monsters. A little tweaking and it could be quite an interesting game.