So..that's how you say AAAGH! in Greek.

An exact translation of AGGGHHH! is in Greek Αγγκκχχχ!

Mostly in Greek war comics we meet AAAAAA!

(The others guys had a lot of fun trying to answer the question in comicstrades.
Well, mostly everyone was saying whatever came to his mind! )

Thank you to the good people at Comics Trades for providing an answer.