As for "reward" the player gets for killing the slime. When we started developing it, instead of "Nothing" the player could get "Shield Skill" that reduced damage taken by 75% for about 8 seconds, but we decided not to add it to the game. We felt that this skill was too boring to use(but it could be really useful).
And.. I forgot to say. Before we published the Carrotest we had tested the last boss for a long time. It was the reason for the high difficulty of the last boss (Our skill was getting higher and higher and it seemed to us the last boss was too easy to pass. In addition, we'd been working on The Carrotest for about three months and it influenced our skill greatly). We were making the shadow more and more difficult. In the end it led us to create an impossible to pass boss.
Before testing we made the boss easier. For example, now the shadow has 1500 HP (instead of 2500HP before publishing).
Thank you very much for your support. We need such feedback, we appreciate this, it motivates us to make more complicated and better games, really.
DONE!