You'd need a lot of shielding. A planet is about the right order of magnitude, though it's still not clear that a human could survive. You wouldn't die of acute radiation poisoning; a few thousand km of rock would be enough to absorb the gamma rays. At 400 AU, you wouldn't immediately boil to death, either. Part of the collision-facing surface of the planet would be converted into plasma and ablated into space, which would insulate the rest of the planet from the worst of the heat.

My guess is that you'd still die from shock waves propagating through either the atmosphere or the planet itself, but it's difficult to predict the exact effects.