@roboneko @Suiseiseki @mimorinka
Native English speaker here.

"hardly raining" always means it's raining very little

"hardly working" is consistent - it means you're working very little

"hard rain" - I would never say this

"It's raining hard" means the same thing as "it's raining heavily" and "there was heavy rain."

There's a band(?) called "Hard Men Working Hard"