The construction is possible, since be is polysemous: so it can be used dynamically when someone deliberately assumes a characteristic or behavior. In this case, “being difficult”suggests intentional conduct.

The sentence is entirely ungrammatical because be is a stative verb and stative verbs cannot appear in progressive forms.

The sentence is perfectly idiomatic, as be is freely used in the progressive aspect whenever duration is emphasized. This usage is natural English.

The sentence should be reformulated using the Present Perfect Simple, because be never carries a dynamic reading. The natural correction is: “He has been difficult for some time.” 

The progressive aspect here indicates passivity. The intended meaning is passive, so the corrected sentence should be: “He has been being treated as difficult for some time.”