In the Manga, Fusion Zamasu doesn't do the weird slime thing when he takes damage. He's fully immortal just like Zamasu was before. Which is how it should have been.
They've also introduced a new twist where Zamasu's fusion will only last an hour, because the earring fusion is only permanent for Supreme Kais, and Zamasu never officially became a Supreme Kai. So because he's just completely immortal, they're basically just trying to draw out the fight until he unfuses.
Supreme Kai also makes a comment about Vegetto Blue's power being greater than Beerus'.
It's so weird when the manga actually makes an effort to apply logic to this series. Zamasu´s fusion being temporary and Goku and co. fighting to draw out the battle until he un-fuses? Sound like a real strategy instead of relying on deux-ex-machina moments.
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