Eh, there's plenty of ways to beat/grind Mihawk without G3 (some entirely F2P, even). G3 helps with a lot of things, but you can go without it.
I mostly want it because I'd like a Luffy unit that I actually use regularly. This game is mostly Mihawk Piece for me.
Duck Squad's no joke. The Vivi Slasher team method will certainly keep you alive til the final fight, but I don't think it's actually ideal: Karue will start hitting you for 11k a turn once he gets below 20% or so. Not even Vivi can save you from that (well, if you had a Zombie team you'd be fine, but I'm not about to farm a fortnight using a zombie team).
I'm thinking Mihawk/Mihawk with a full team of INT slashers. Might bring along GP Usopp too, but one extra turn via Mr. 3 should be enough to burst the last of Carue's health, especially if you save the Mihawk specials for the end. Gotta find slashers with good RCV though. Croc, Mr. 3, and Helmeppo might be enough.