This has a neat mechanic. It took a bit to get the hang of throwing the rocks well, as I often waited to long to release.
I like the concept, though I think the mana system could use a bit more refining. When you have full mana you can throw 2 rocks, at full speed/power, regardless of the displayed mana level, but then need to recharge to full before you can throw again. It felt like the mana level didn't matter and could have simply been 2 dots. If you are going to have a mana level displayed and fast throws use more, then It would be nice if the remaining mana level affected the power you can throw with, and you could throw any time you had mana. I know in the dev log you mentioned trying to reduce the ability to throw constantly. If you could do many 1 dot throws that might be too much, but the mana level system just does not feel like it conveys the current gameplay well. If you want to stick with needing to recharge for a certain time before throwing, I think it would be better to break the recharge and available throws apart. Have 2 dots for the throws, and a progress bar showing the current recharge state for getting the next throw back. This would allow a certain amount of recharge time being needed per throw, but also allowing the player to be able to recharge just one at a time.
I managed to play through all the levels. The first mass skeleton level was tough at first until I learned how to use the rocks well for blocking, though still sometimes had an arrow slip through at an angle I did not expect. Overall this was fun and a great start.