Well, this is where the subject may be "subjective." This seems to be about Hercules, the mythical Greek hero. Therefore, there is a poetic element here that informs the word choice. It wouldn't be wrong to use "slung a spear off far into the distance" elsewhere, but it would seem more awkward in a more mundane context. In particular, the word "slung" is not a typical one.
The word "off" in "off far" seems to insist to me that this is being thrown a great distance, but those two words won't be used in isolation. It could be
Hercules slung a spear off far towards a target
Hercules fired an arrow off far into the distance
"Far off" is much more common:
On a clear day, you can make out Haleakala far off in the distance.
You could, therefore, switch "far" and "off:"
Hercules slung a spear far off into the distance...
But I will emphasize that in this poetic context at least, the grammar is flexible and not necessarily wrong; I prefer this last sentence to the original one, but, the original one has a different emphasis, it's "quicker" and less expected; the event occurs in the sentence almost before you've read the words that made it happen. Perhaps it emphasizes Hercules' strength; I don't know.