Well first, I tried to level up all units at once, but they kept dying in one hit from almost anything. Not to mention that only the blue slime would ever attack.
Then, I decided to focus on the blue slime, but no matter how much I level it up, it can never kill anything. And focusing on numbers doesn't help, either. The best I can do is raise HP once or twice to have the blue slimes survive more than one attack, but no other stat raising seems to do anything. It's only after the 15 goal that leveling up seems to do anything.
Like, are the other two units even able to ATTACK at all?