The Immortal God Zamasu survived Sword Stab And Final Flash From Goku. In the history of Dragon Ball, we have one other immortal villain- Garlic Jr.
In Dragon Ball Super Episode 57, we see The Immortal God Zamasu like never before. He is immortal in a badass way. That is he can’t be killed or even harmed from what has been established till now! SSJ2 Trunks got the better of the fight in multiple occasions Stabbing Zamasu with his sword and throwing a full powered Final Flash. Zamasu took both the attacks head on without getting hurt and the minor wound in his body healed in no time. This episode makes it very clear that Zamasu can’t be defeated just by getting powerful. As later on he holds Goku and Trunks while Black charges up a ‘ Rose Kamehameha ‘. He takes the attack head along with Goku and Future Trunks. Even though Trunks and Goku get totally wrecked, but Zamasu remains absolutely unharmed.
So, at this point in time, it is absolutely clear that getting Powerful or attacking him repeatedly won’t get the job done. So, Goku- Vegeta fusion or a new God form would not be enough to take him down. As of yet, we don’t have any good clues to figure out how all this might end or how to connect the dots between Present Zamasu, Future Zamasu, and Black. Just to clear out the confusion, In episode 57, after Goku and company returned from the future we still see Zamasu is in universe 10. Indicating there is actually 2 Zamasu future and present. However, there are theories going on like they are actually the same it could be some kind of time paradox or time difference between 2 universes. It could also be that the last scene of episode 57 is a flashback that is Whis or Kaioshin is seeing how Zamasu went to the Future. As we know from Episode 58 preview, Whis is aware of Zamasu threatening Zuno. Anyways that’s a topic for another discussion.
So the problem we have in hand currently is the immortal Zamasu that is causing terror in the Future. To defeat him the interference of the Gods might become a necessity. Something along the line of sealing him down could be a solution, but we don’t know for sure. Though, Toriyama declared The Omni-King will be involved I think they will try to avoid him as long as possible, and since Beerus don’t want Zeno to find out anything about this he might be the first one to get involved.