On solar radiation they could. Also Superman doesn't need to build up an immunity. If you punch a tank 50 times over 50 years it will do as much damage as if you punch it 50 times in 50 seconds. Now replace the tank with Superman and you with Krillin. Keep in mind Superman also can do some Jedi mind tricks. Albeit it's closer to telapathy/telekinesis. But a thing I've noticed is that you keep referring to the rest of the Sayians as if they were at the best condition. And if they can then the Kryptotians can be in tip top shape too. Now imagine an entire species of solar-radiation-pumped people with their technology too, (because they were rather technologically advanced) coming straight at the Saiyians. And the above example is in the context if either species hadn't been nearly eradicated.