What are possible methods to beat a goblin clone attack?
There are a lot of cards to counter this move, like Arrow, Poison, Valkyrie, Electro Wizard, even Mega Knight.
However, I think it is best to "read" their moves and spending elixir efficiently. Goblin Barrel takes time between being thrown until it lands. If the direction is to your King Tower, then be ready to counter it. And it is expensive to launch Goblin Barrel + Mirror + Clone, so they must have been holding cards for a while. If you play it right, you shouldn't die in their first move.