Concept is good! I've always enjoyed these type of survival games. For the the difficulty curve between level 2 and 3 was a bit much - I managed to cut down 6 sags before dying in my final attempt. Without being able to heal back, I think it was meant to be a level where it's impossible to finish? If so maybe a timer/scoring system would push the player to try harder or strategize more! Keep working and learning, you're doing great.