‘Hold my beer’

"Hold my beer" is an informal phrase used to signal that someone is about to attempt a reckless, foolish, or surprisingly daring action. Formerly the idiom was “hold my coat!”
It implies, "Watch this," often preceding a stunts or a challenge, and acts as a humorous prelude to a potentially disastrous or impressive feat. Suggestions from various internet websites are the literal κράτησε την μπύρα μου και δές. Οr Έλα να δεις" "Wait till you see this". Also the common phrase "κάτσε να δεις": Just you wait and see" to a person who has challenged you to execute something daring or reckless.
The sentiment goes back as far as the Greek of Hipponax of Ephesus' famous threat to the sculptor Bupalus (Fragment 12) :-

<< λάβετέ μεο ταἰμάτια, κόψω Βουπάλου τὸν ὀφθαλμόν.
ἀμφιδέξιος γάρ εἰμι καὶ οὐχ ἁμαρτάνω·»
“Hold my coat while I belt Boupalos in the eye.
I am ambidextrous and never miss a punch.”

But there are several Greek versions of the above text.

In my experience, this one has more or less made its way into worldwide chat and messaging jargon as is…

But AFAIK it’s not just an alternative to Watch this / Wait till you see this…
It’s more of a somewhat playful response to an unsubstantiated boast, coming from someone who actually is superior at the quality in question…

Examples:

Apple released the iPhone Air last fall, pitching it as their thinnest phone to date, but in the cellphone market as a whole, it was actually the 21st thinnest phone out there… Hence, one could comment something along the lines of:
Apple: This is our thinnest phone
Rest of the market: Hold my beer

Or, if, say, Germans are known for their punctuality, Swedes be like, Hold my beer, meaning that they are in fact far more punctual…

Or, if I was to present myself as a seasoned linguist, most of the members of this forum would be like, Hold my beer…

Possible (albeit imperfect) ways of conveying this nuance in Greek could be:
Μίλησε κανείς για [λεπτό ντιζάιν / συνέπεια στα ραντεβού / γλωσσικές γνώσεις];

…or (provided that the rest of the context allows for some liberty) far more opaque variants such as:
Εδώ γελάμε…
Δεν πα’ να δεις αν έρχομαι;
Πού πα(ς) ρε Καραμήτρο;

Disclaimer: My answer is subject to revision, and is mostly provided just to get the ball rolling…

For some of the further examples provided by m_a_a, perhaps you could also use expressions such as σιγά το πράγμα and σιγά καλέ.

What marvellous answers! It may be to start the ball rolling [για να βάλω το νερό στ’ αυλάκι?] but to me they are a mine of information as I follow them up.
(I had a terrible Greek lesson today, trying to communicate on all sorts of subjects with speakers at all levels of spoken Greek. It was an experiment but, under pressure, stuff I could normally easily cope with produced brain fog and brain ache! A salutary but necessary humiliation, as αυτό το μάθημα ήταν για τα πανηγύρια, I thought, like a deer at bay! These replies are a real antidote to my downheartedness.
Thanks, as usual, to cougr but special thanks to m_a_a for all the examples given and all the illustrations. I’m glad slang.gr is up and running again, for the detailed background to Πού πα(ς) ρε Καραμήτρο; except that that πάρτον κάτω puzzled me as in English ‘take him down’ is the phrase that judges use to prison officers in court to take a guilty man down to the cells!