The game concept is really cool and seems like it could be easily tailored towards speed running due to the heavy emphasis on the time mechanic. I enjoyed the wave based survival format with a different boss each round, it works really well, especially for a game jam game! I also really appreciated that you put a button in the lower right hand corner to explain how to beat the boss. That's a super helpful feature and helps majorly to increase the accessibility of the game.
I couldn't get past level 2 unfortunately, I could get the shield down on the monster but couldn't jump on it as it was always in the lava.
If you wanted to improve, I suggest opening up the screen more, maybe have the platforms in the middle rather then up top. Also, falling in the lava could respawn the player with damage instead of instant killing them and sending them back to the main menu.
Overall cool game, I would love to see you expand on it some once the jam is over!