Winning by a landslide basically means winning by a lot. Shellacking is a slang word meaning to beat someone physically or by a lot of votes in an election. I'm assuming they're talking about politics? It means that winning by a lot was an understatement. They received so many votes that the word landslide (to gain the majority of votes in an election) didn't compare to how much they won by - how they beat the other team/party by. Hope this helps :)

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