Why should alcohol be part and parcel of this hedonistic party? Despite what Corinthians says, there are other ways to have fun which do not impair next-day performance.
Namely, sex. Sex trumps alcohol any day. So make sobriety a prerequisite for sex and you're good.
I'm assuming mainly male, generic army. Dudes like to drink, get raucous, rage, chase ladies, and love ladies. If that's what you're working with and you're willing to get dirty, bring in strumpets, invent a system where only sober men may romp with them in specialized tents. Doesn't seem like a great option to me because all but the raunchiest most male readers will find this unsympathetic.
Better yet, bring in non-prostitute women: Either the women join their men to fight in this last-ditch battle, sending the kids off with the old people, or the families join the party from the surroundings. In either case, only sober couples may go off and couple in tents or the woods or what have you.
Another mechanism: Make an economy of alcohol in the party that limits overall consumption. Soldiers, like anybody, have hidden talents besides their ability to kill people. They exchange entertainment (playing an instrument, singing a song, juggling, telling a story, acrobatics, feats of strength, wrestling, swimming, running jumping over the campfire, winning card games) for alcohol. You get most for winning but even losers get a modicum of mead or ale. This slows down and limits the drinking. Even the most talented men (or women) will falter in their ability to impress as they booze up. Start the party early in the day to start outdoor activities and then you can end it earlier.