Are both "wipe us out of existence" and "wipe us off of existence" idiomatic? To me, the first one seems to be more correct, but the second one doesn't seem to be wrong either, because "wipe off" also means "removing", so "removing us from existence". However, I am not sure if the second phrase is idiomatic even if semantically it's correct since I am not a native speaker. Is it idiomatic?
"Wipe us out of existence" is fine, but I believe you're confusing the second with the idiomatic expression:
Wipe us off the map / off the face of the Earth
You can use almost any geographical location with this idiom:
The Empire tried to wipe the Rebels off the face of the Galaxy, but they used the Force.
Also, "wipe off of existence" doesn't really make sense. "Wipe out" means to eradicate (remove completely) while "wipe off" means to erase (delete markings from some surface).
I mean, yes, you can say:
Erase us from existence
but this is the meaning of "erase" that's closer to "obliterate".
Side note: Some days, you can't help but wonder why even simple English words have so many different meanings.