Well, I played more...and I can tell you that two units are useless and the third one is only usable after 15 runs. The slingshot never shoots anything and the red slime does nothing. Meanwhile, the blue slime only gets good enough to kill something (by ganging up on a single enemy with a full team of 5 or 6) after around 15 runs.
So reaching the 15 run goal is impossible unless I get REALLY lucky with exp sales.