Okay...
I think that from your current team you should definitely keep Delphox. It has a decent moveset right now, but I'd have it learn Heat Wave from a tutor in Peridot Ward instead of Flamethrower so it hits both opponents in the Radomus/Serra battle. I'd also switch Dazzling Gleam for Will-O-Wisp if you have the TM for it, but if you do not, then fairy type coverage is still great especially against Luna. Mystical Flame is also an amazing move since it always lowers the targets Sp.Attack by one stage.
I think you should also keep Zoroark (it's my fav mon, i might be a little biased, but...). It has great potential for easy set up thanks to Illusion. I'd replace Low Sweep and Foul Play with Nasty Plot for set-up and Grass Knot/Thunderbolt for coverage. It doesn't always work, but it can definitely help you ace a few battles, maybe this Gauntlet of Hell is one of them.
From your box you could utilize the Alolan Vulpix. If it doesn't have Snow Warning as it's ability you can buy Ability Capsules at one of the stalls in Agate Circus. It could up your chances a fair bit if it set up Aurora Veil (that strat is insane, trust me). It also rocks attacks like Blizzard and Dazzling Gleam since they hit two targets in doubles. Having it learn something like Thunder Wave from a TM for additional support wouldn't hurt either.
If I were you I'd really pick up that Larvesta and evolve it onto Volcarona. That thing is an absolute beast when you let it use Quiver Dance one too many times and it can sweep whole teams. I lost count how many times it carried me through really hard battles in post-game. It even has a wiki page dedicated solely to the special Larvesta encounter and it isn't that hard to obtain though I recommend saving before trying to catch it, just to be safe. A perfect moveset for Volcarona is Quiver Dance (obviously), Heat Wave, Bug Buzz and Morning Sun/Sunny Day.
As for the EV/IV conondrum, I myself am not a scholar in this field (lol), but I think I understand enough to use it to my advantage. EVs can be gained from fighting different Pokemon which are worth from 1-3 EV points in a different stat (Defense, Speed etc.). Maximal value in one stat that you can gain by beating other mons is 255, and you can fully train only two stats for one Pokemon (so you can gain up to 510 for Pokemon). You can split the points between different stats, but they should be concentrated around the most crucial ones depending on the mon. For example:
For Zoroark you should have 252 EVs in Sp. Attack, 252 EVs in Speed and 4 EVs in HP (at least it's the build I used)
For a tank you'd have 252 in HP and Defense/Sp. Defence. Items called Reset Discs can be used to get read of already gained EVs.
If you really feel like you can't beat the Gauntlet you can watch strats on YT, there are a few videos where people use pretty rad strategies to beat that monstrosity. I hope I was at least a little helpful. And i'm sorry for my word vomit. Hope you manage to win this :3