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Try a few things mentioned here: https://gamefaqs.gamespot.com/boards/221208-kirby-battle-royale/76253547
I was able to get it by using Doctor's yellow potion to KO everyone in the last few seconds so they couldn't launch. If you set the CPUs to level 2, they will be less likely to interrupt your charge attacks, but they won't make a mad dash for the rockets in the last 10 seconds either.