How did Zamasu defeat Beerus?

GoDs are connected to Kais, so once Zamasu killed the Supreme Kai, Beerus dies as well. By killing the Kaioshin whose life is linked to the GoD you can kill them indirectly, in Future Trunks timeline during the fight against Dabura and Babidi the Supreme Kaioshin died, so Beerus died with him.

What episode does Zamasu defeated?

Let’s just make one thing clear here: Future Trunks defeated Fused Zamasu in the last episode. He sliced the arrogant, immortal guy in half and what happens in “Farewell, Trunks!” by no means negates the fact that Trunks is the hero of this story arc.