So one thing I see right off the bat is that you have two different weather setters on your team. I’d suggest that if you’re going to have weather on your team at all, you should make sure it’s doing something for you on a strategic level. It doesn’t seem like your team is built to take advantage of either sand or night in particular, though half your team is immune to sand so that’s a point in sand’s favor. I would suggest leaning into either sand or night or maybe reevaluating weather on your team altogether.
My next idea is influenced by my own playthough on hard difficulty. I found that the e4 when I faced them required that I not only have a good team with strong pokemon, but that I had some kind of strategy to execute. I have not attempted the elite 4 rematch, so I can’t give advice on those fights specifically, the first round of e4 I found I did mutch better when I had a strategy even if that strategy only a rough idea of what each pokemon on my team was supposed to do for me.
To that end I would suggest you evaluate what kind of strategy you are attempting to execute. Currently it seems like your team sort of has three things going on, you have some weather stuff going on(spiritomb, tyranitar) , some very offensive pokemon (lucario, m.d. metagross, d. haxorus) and a couple of more bulky attackers, those being your armored pokemon. Given that d.volcarona is a must for you there are a few strategies that you might consider, in the order I thought of them:
Idea 1: Stall
I ran the e4 the first time with a stall team based on ORAS OU’s mega sableye stall strategy, and it worked wonderfully, except that I felt that I didn’t have a good toxic spikes setter, a role that d.volcarona fits perfectly with its acess to toxic spikes and roost, as well as it’s huge speical bulk. If you were to create such a team there are a number of good resources online such as this one.
As a brief overview, the team revolvs around mega sabelye to reflect entry hazards and a plethora of other annoying moves back at your opponent, and wittling down the opposing team with walls. For such a team I would reccomend d.volcarona as a toxic spikes setter, defogger (to remove opposing entry hazards if they get up), and special or mixed wall, m sableye as itself, chansey as a special wall and stealth rock setter, clefable as a wish passer/cleric and pysically bulky unaware wall.
As for the last two team slots those are dealer’s choice within a couple of parameters: you will probably want another physical wall, and you will probably want these last two pokemon to have roar,whirlwind or dragon tail, and spikes between them and defog or rapid spin if you don’t want to use defog on d.volcarona. As a final note it is important that almost every pokemon on this team have access to some kind of reliable recovery (like recovery, roost) since you will be winning throuh attrition and smart switches, rather than in a more conventional manner. This team style works espeically well on hard difficulty when you can’t use items in the e4 fights.
Idea 2: new moon/night
I run a new moon focused team in post game and its a blast. D. volcarona having access STAB lunar cannon/dark pulse and it being hard to take down makes it an interesting choice on a team that is looking to abuse the new moon weather condition. I would build it as a speically bulky brawler who throws out nutty dark pulses when night is up, or as a scarfed/specced lunar cannon spammer with absolution as its ability. The fact that it has no weaknesses also means that it doens’t technically count towards this team style’s propensity to be weak to faries. This weakness is, however, somewhat countered by fairy moves being weaker under night.
For team reccomendations I would say that spiritomb should be your primary night setter and lead. I run a phsycial revenge killer/utility set with dark rock as held item, sucker punch will-o-wisp protect and pursuit, though I think that the 4th slot would be better as knock off. No matter what set you decide to run building spiritomb with max hp investment it important to increase its longevity. If you run only spiritomb as your night setter then I would even suggest investing into defense/special defense and relegating it to more of a full support role.
That brings me onto my next idea about this type of team, which is that if you have access to m.d. charizard you could run it as a secondary night setter and stallbreaker. With access to taunt, and roar and being able to set nght for STAB shadow balls, m.d.charizard has the tools to break all but the most stubborn of special wall, on top of being a superb backup weather setter. Just don’t run lunar cannon on d.charizard,it doens’t add any coverage and is weaker than shadowball under night since charizard doesn’t have dark STAB.
If you lack m.d. charizard then I would suggest either mega shiftry as as a physical shadowdance sweeper, or m.d. typhlosion for a slightly less fragile, special attacking option. m.d. milotic also is a good bulkier mega option that abuses night on the special side, by throwing out absolution boosted attacks. Beware though that d.milotic’s only two weaknesses are also boosted under night. If you feel that there is another mega that better serves your conception of the team that is not a night abuser then by all means use it. It would be your team after all.
For another specific pokemon idea, delta roserade is rediculous on this kind of team. With shadowdance and max speed invenstment it outruns everything I’ve had to deal with so far, which lets it spam choice spec boosted, stab boosed, night boosed lunar cannons. That’s an effective base power of 212 coming off of a respectable 125 bse sp. attack. 187 base sp. attack with choice specs. However, given that you are using d. volcarona that might be too many pokemon who are interested in spamming dark type moves. A physical attacking giratina could also find a home on this team especially if you don’t have access to m.d. charizard since night lets shadow force go off in one turn and boosts it given its a ghost type move.
A couple of general reccomendations for this team are
1: have a dedicated fairy killer. I use durant for this personally and as a physical sweeper, since hone claws boosts accuracy and attack by two stages at night. I think thought that there is a better choice around, probably one that is more specially defensive. Which honestly sounds like a perfect job for volcarona.
2: Having a rapid spinner or defogger can really help out, especially when you run into the rare opponent who likes to lay down livewires. I use tentacruel for this job since it can also lay down toxic spikes and can put hurt on many things thanks to new moon boosting surf. Again d. volcarona is a shoe-in for this role if you don’t run it as a lunar cannon spammer.
3: Having a night abuser that is not a primary dark or ghost type attacker is good for coverage reasons. m.d. typhlosion fits this bill nicely as does any pokemon with access to hone claws or surf, given that both are boosted under night.
4: This suggestion goes for most teams, it is important to have at least a couple of bulky pokemon on the team to take hits and pivot around. Again, d. volcarona can fit this role with armor becuse of its high special defense and decent defense. Make sure that you have both at least one physically bulky and at least one specially bulky pokemon on the team.
Once all this has been considered, the final slots are there to help shore up any parts of the team you feel need more work.
Idea 3: balanced team
This is sort of the cannonical pokemon team where you just create a well rounded team with lots of coverage and such. for this style of team it is often reccomended to have something along the lines of two attackers, two tanks and two supports represetned in your team. Bear in mind that a single pokemon could potentially fill multiple roles. This kind of team is the most open ended I feel. I feel also that being open ended it has maybe the most potential to just fill the team with pokemon that are known for raw power. This includes many legendary pokemon, aegislash, speed boost blaziken, and mega metagross to name a few. Here are some suggestions for this kind of team:
1: Have a good defensive core.
That means that the pokemon you consider the ‘core’ of your team, so to speak, have complementary typings that cover each other’s weaknesses. Common examples are the fire-water-grass core and steel-dragon-fairy core. Here is a more in depth guide to three type cores. I should note that probably not all balanced teams take this approach. It’s just one I’m familar with for building balanced teams.
2: Again having a pokemon that can defog to remove entry hazards is a wise idea. Again d.volcarona provides this utility very well.
3: To finally address your concern over the lack of ice/fairy moves: I feel this is the type of team where type coverage really gets to shine as a core part of your strategy rather than simply being an important aspect of teambuilding. D.gardevoir is a powerful, although frail, attacker with bolt/beam STABs and access to moonblast, though running scarf can feel important given its merely decent speed. It also is a fine candidate for the mega slot. On the defensive/support side clefable provides STAB fariy coverage while also being able to check set-up sweepers thanks to unaware, and can pass wishes, and can learn heal bell. It also could form part of a fairy-steel-dragon core. Standar gardevoir is also a stand out in the fairy department, especially its mega form.
4: with two of the e4 being weather based teams it might not be a bad idea to have a pokemon who is able to overwrite unfavorable weather conditions. Rain/night are considerations since they are not represented to my knowledge in the e4. However, since many weather setters in these catagoies are somewhat frail it may be better to try and remove opposing weather setters and wait out the unfavorable conditions.
Idea 5: hyper offense
I’m just sort of throwing this out there as another idea, though I am the least informed about this style. It is my understanding that the central idea is to pick a bunch of fast pokemon that hit like trucks and have good coverage between them and go to town. This strategy has the potential to be quite frail, though if you are running the e4 with the switch battle style, this type of team can be very powerful since you can ensure you generally get favorable matchups.
For d. volcarona in this team style I might reccomend running choice scarf since the idea of this team is to always be going to KOs right out of the gate. D. volcarona’s high special attack, good speed, and interesting coverage options lend itself decently to this team style.
Those are my ideas about beating the elite 4 rematch including d.volcarona on your team. Let me know if you want to discuss anything I’ve said here further, I live for overanalysis.