"That singer sings well."
Translation:Penyanyi itu menyanyi dengan bagus.
Halo teman! I think the form "dengan baik" is grammatically correct here. Because it is indicating the adverb that modifies the verb. And, about the drop of "dengan", when used by natives, I think it is possible for the colloquial speech. Since I am not a native speaker, I cannot assure that. I can only say that I would prefer the sentence with "dengan baik" (well) here. Please try using the reporting tool to add your sentence, then the collaborators can decide if the translation is accepted or not.
Selamat belajar! :)