Badly.
I miss you terribly, would be better.
No, it should be "She sings badly."
No. The sentence should read "She sings badly."
Yes, "she sings badly" is grammatically correct. The adverb "badly" is describing the verb "sings" in this sentence, indicating the quality of her singing.
badly, horribly, unbelievably, extremely, seriously
very, extremely, terribly, exceptionally, greatly, immensely, exceedingly, badly, woefully
No, it's an adjective. The adverb form is terribly.
That is the correct spelling of the adverb "badly" (poorly, or fervently).
my brother behaved badly with me
It is a colloquial phrasing, because you are using "really" to mean "extremely" and "badly" to mean "desperately" or "greatly." A less colloquial form is very badly.
There are many better ways to say this, such as "You're not a (very) good painter" "You don't paint well" "You're a bad painter" "You painting lacks (what it lacks)" "You paint horribly" But "You paint badly" works.
badly hope i helped!