The earliest MSS (R, V) transmit 246 with faulty metre:Footnote 1
In place of ὦ the recentiores (PCH, LB, Ald) give ἰώ.Footnote 2 The resulting sequence—tribrach in the second ‘foot’, anapaest in the third—is uncommon, but there are several uncontestable instances (Ach. 47, 928, Nub. 663, Av. 108, Eccl. 315).Footnote 3
Some recent editors, however, restore ἐπιτετρίψεσθ᾽Footnote 4 from the lemma of ΣR 246b, objecting that: (i) in Attic, while the future middle of τρίψομαι is attested in a passive sense (Thuc. 6.18.6, 7.42.5), compounds attest only τριβήσομαι (see LSJ s.v. τρίβω);Footnote 5 (ii) the future ἐπιτρίψεσθ᾽ does not suit the perfect participle καταμεμυττωτευμένα at 247—as ‘a simple future would imply, nonsensically, that Megara will undergo “crushing” after it has already been “made mincemeat of”, whereas in fact the two verbs refer to the same operation’.Footnote 6 Metre forbids ἐπιτετρίψεσθ᾽ with ἰώ, but allows it with ὦ.Footnote 7 Yet ἰώ, not ὦ, is the appropriate word. The speaker (Polemos) delivers a sequence of four interjections, the first, second and fourth introduced by ἰώ (236, 242, 250); to introduce the third with ὦ would interrupt this pattern.Footnote 8 Moreover, the future perfect (-)τετρίψομαι does not appear to be attested with certainty in Classical Greek. Μέγαρα Μέγαρ᾽ has been judged dittographic; Elmsley (n. 4) proposed ἰὼ Μεγαρῆς, ὡς ἐπιτετρίψεσθ᾽ αὐτίκα with ἁπαξάπαντες καταμεμυττωτευμένοι at 247,Footnote 9 Sommerstein (n. 3), 358 ἰὼ Μέγαρ᾽, ὡς <ξυν>επιτετρίψεσθ᾽ αὐτίκα,Footnote 10 adopted by Henderson 456.
I suggest a different approach, replacing the future or future perfect with a perfect:
‘Now you are done for’. For this idiomatic perfect (KG 1.150) in combination with αὐτίκα compare 366–7 ΕΡ. ἀπόλωλας, ἐξόλωλας. ΤΡ. εἰς τίν᾽ ἡμέραν; | ΕΡ. εἰς αὐτίκα μαλ᾽ … The perfect verb ties in nicely with the perfect participle which follows. Restoration of a second-person singular, in place of a second-person plural, is commended by the fact that Μέγαρα is found elsewhere only with a singular verb (Thuc. 1.114.1, 4.70.1; Dem. 4.9; Strab. 8.1.2). Corruption to a second-person plural future will have been aided by the use of the second-person plural future in the first and second in this sequence of interjections (236–7 ἰὼ βροτοὶ … ὡς αὐτίκα μάλα … ἀλγήσετε, 242–3 ἰὼ Πρασιαὶ … ὡς ἀπολεῖσθε τήμερον). The switch to a perfect, in the third, is followed by a further switch, to a present, in the fourth (250 ἰὼ Σικελία, καὶ σὺ δ᾽ ὡς ἀπόλλυσαι). The same form appears at 369 καὶ μὴν ἐπιτέτριψαί γε.