Well, all you need is to keep increasing the costs past f1, and include a few current f1-stompers at the beginning, with their reproduction turned off. Start with something like just 5 fruitflies and 5 sabers, make sure they can't reproduce, and once they kill them, increase to 10 fruitflies and 10 sabers, then 20, etc... Ultimately, you'll have an ex-nihilo bot that can kill 200 fruitflies and sabers at once- then it's ready...