How strong would one have to be to one-shot their opponent? Raditz was like, 3x or whatever stronger than Goku but he didn't one-shot him, so I'd assume it'd have to be like 10x or 15x but I'm not sure.
Raditz did kind-of one-shot Goku at the beginning, before he left with Gohan. I don't think Raditz was out to kill them straight-away so he prolonged the beating. (He even said he'll make his attacks gradually stronger or something.)
Goku also one-shotted Recoome at above 60,000, but below 90,000.
Purely plot dependant. Based on the gaps that the manga suggests, 1.2-1.25x seems to be the bare minimum, whilst a Several-fold gap is more of the standard.