Loki: I have an army.
Tony Stark: We have a Hulk.
I just can't get why Stark uses an indefinite article with a real person in a phrase "We have a Hulk", is this just Stark's sarcasm or something else.
The word here is often seen in the idiom as well. But not always.
The a is used with names in the expression: "We [or other pronoun] have an [name] here" to make fun of or be sarcastic about a person.
The idea is that the person about whom this is being said is acting, in the eyes of the person who says it, like someone much grander than he or she is.
It is part of Starks sarcasm. He is treating "Hulk" as a tool. He is objectifying Hulk.
It is fairly natural as "Hulk" is not just a name, it is a description of the character: Hulk is a hulk. This makes objectification easier. It would sound odd to say "We have a Nadine". It means that Stark is following in parallel Loki's expression "I have an army/We have a 'Hulk'"