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What the two other solutions neglect to tell you, is that not only do you have to crash into the walls 30 times, stuff has to be bounced off your katamari. So rolling into a wall before you've rolled over anything won't do you any good.
I recommend doing this on a time trial since that's basically endless too, plus you probably have access to time trials before you have access to eternal modes. The "Chatéau Notre Desir" solution also doesn't work as once the game stops asking for memories, you've most likely grown way too big to be able to crash into walls anymore.