This challenge stumped me for a while, but I finally tackled it today. My genome got a lot more complicated than expected, but it went something like this:
I started with an initial glueocyte to suck nutrients from the existing cells and get in the way of their swimming. But it needs to move around, so I made it split into a devo-keratino swimmer. Now it can survive and eat, but sometimes has to swim a long way to find other cells to eat, so I added a phagocyte, allowing it to snack along the way. So at this point I have something very similar to Alast's diamond-shaped organism, with two sources of food.
How to reproduce? Well, spit out more glueocytes of course. They do a great job of gumming up the works! But then there are those darned keratinoswimmers, which are immune to devocytes, and won't stick to glueocytes, and are just hardy enough to make it nigh impossible to out-compete them on food alone. What better way to deal with them than virocytes that will hijack their reproductive cycle? So I modified my four-celled diamond swimmer to spit out both glueocytes and virocytes. I ended up using two different modes of glueocyte so I could adjust the balance, and increase the number of virocytes.
It got tense toward the end--more than once, I saw the original cell population drop below 10, only to shoot back up over 100 again. But finally, they all died!
The best part, though, was when I did "Tip the Balance II", using the exact same genome, placed my one cell, and solved it on the first try!