how about "duramente" ?
In English, Hard can also = Tough "He is hard!" "Hard as nails!"
Is it the same in Portuguese?
We don't say like that in Portuguese. We have, for instance, "ele é cabeça dura".
Would it never be "Eles trabalham duros"?
Here "duro" is working as an adverb (modifying "trabalham") and adverbs are invariant (do not vary with gender or number). As an adjective it does vary: "dur/o/a/os/as".