‘Hold my beer’

[Theseus]

"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.

 

[m_a_a_]

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…