John 10:33 in the Greek reads (NA28):
ἀπεκρίθησαν αὐτῷ οἱ Ἰουδαῖοι· περὶ καλοῦ ἔργου οὐ λιθάζομέν σε ἀλλὰ περὶ βλασφημίας, καὶ ὅτι σὺ ἄνθρωπος ὢν ποιεῖς σεαυτὸν θεόν.
There is nothing special or difficult about the Greek here. A literal rendering is:
The Jews answered him: We are not stoning you for a good work, but for blasphemy; because you, being a man,1 make yourself God.
[This is a purely linguistic question: it has nothing to do with theology whatsoever.]
As I understand the use of the accusative here, it would seem the Jews are accusing Jesus (no pun intended) of identifying Himself as 'God [Himself]' (θεον) as opposed to 'a god,' (θεος);2 that is, 'as to his nature, θεος.'3
A simpler way of putting it would be that the syntax of σὺ ἄνθρωπος ὢν ποιεῖς σεαυτὸν θεόν should mean the Jews are saying 'You are claiming to be God [Himself] whereas you are a man [as to your nature],' and not 'You are claiming to be [as to your nature a] god, whereas you are a man [as to your nature].'
Or simpler: they are not saying 'you imply your nature is god, but you are only human,' but rather 'you imply your are God [Himself], who are a [mere] man!'
How grammatically accurate is the above assessment?
1 Or 'who are a man' or '—a man—'
2 Obviously not in a polytheistic sense, but insofar as God is 'θεος' in the essential sense, qualitatively speaking.
3 Cf. Jn 1:1c