My reaction after the first couple levels: "Aw, they're so cute, why am I just murdering them when they're just walking on the street even?" My reaction after the last couple levels: "WHY DOES THE UNIVERSE HATE MY LAWN?!?!"
I'd like if there were checkpoints of some kind. When you spend 20 minutes and die on one of the last levels and you have to do it all over again, it feels tedious. Maybe let you restart on level 40 with a point loss?
I finally fixed the invincibility bug. It wasn't what I thought (although that problem did break the entire game for some reason when I went back to fix it), but it was because of running into bosses and trying to calculate your health in a bad place.