This challenge can be difficult or easy, depending on your choice of substrate settings. A few of the challenge substrates make it quite easy to evolve successful swimmers from scratch, and I have done it dozens of times on various substrates.
The best one that comes to mind is "Tip the Balance", due to its large size and high nutrient concentration. Long-tap the challenge to open in experimental mode, and sterilize to start clean, or just download this empty substrate:
https://www.dropbox.com/s/9w5qf6rji6rux ... trate?dl=0
Enable only phagocyte and flagellocyte, turn up radiation a bit, incubate, then "Contaminate with random cells" until you see one or two organisms begin to take over. Turn off contamination and evolve some more to get a stable population. The longer you evolve them with radiation, the more efficient and streamlined they will become.
Sometimes I have gotten a viable organism with only the random contaminant cells, and no evolution whatsoever. It is such a rich substrate, swimmers have an easy time.
You can use the same process on the "Macrophages 3" challenge, but it takes considerably longer due to the smaller substrate size and less nutrients.
Even the default "New Plate" settings are reasonably good for evolving phago-swimmers. I have found radiation around 0.010 to 0.020 be a good baseline for moderately fast evolution, maybe dropping to 0.005 or lower for fine-tuning.