Whose to say that instant transmission wouldn't be able to get Goku away before the oracles could do anything, or even out of the past portals or future ones? Also, Goku wouldn't have to charge a small enough blast to obliterate Atheon. Honestly, he could probably just fly forward and rip Atheon apart with his bare hands.
Edit: Sorry, seems like a double post.
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My knowledge of DBZ universe isn't the best but could be yeah. We can't really know how it could turn out because in real life, Atheon wouldn't wait, he would just teleport him immediately. Then again on the other side we have Crota and by the time it takes Goku to figure out how to kill him, Crota would spam his oversouls and try to kill him.