Thanks for the feedback!
Yeah, the healing is definitely too slow. I was planning on making it so that the more slimes you gather, the more healing you get from the slime hive mind, but I didn't get around to it unfortunately. You didn't miss anything at all. Getting more slimes increased your max health and healed you for that amount, but it still wasn't enough to counteract the amount of damage you were likely to be taking.
I'm glad you liked the music and art!
I was a bit worried about camera shake being too much for how frequently a player is both dealing and taking damage. I could probably implement a critical hit system and have the screenshake happen when one of those pops. As for particles, there's not a built in particle system to my knowledge, but getting a simple one made isn't too big of a deal. I agree, it would make it look much cooler. As for hit stop, I don't know, there's not really a whole lot of animation involved in the attacks so there wouldn't be a great place to put it, unless I put in different kinds of attacks, which I'm not necessarily opposed to as an idea.
Don't worry, I didn't take it that way! Constructive feedback is always good. I did want the game to be difficult, but I think if I implemented levels or something, with some smaller rats to deal with until Ratnir was the final boss, it would be better for getting a player into the flow of the game, and help them feel more prepared for the challenge.