Hi. This was the first time I built my own Nat Dex Ubers team and wanted some feedback.
Lucario-Mega @ Lucarionite
Ability: Justified
Tera Type: Normal
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Close Combat
- Meteor Mash
- Swords Dance
- Extreme Speed
I picked this mainly because I like it. Extreme speed is good priority and helps pick off weak mons reliably. It is a good speed tier in order to naturally outspeed a lot of bulky mons but is super frail. I have been thinking of replacing swords dance since I almost never use it in battle and I get the attack boost from meteor mash more often. Close combat is the highest damaging move.
Zacian-Crowned @ Rusted Sword
Ability: Intrepid Sword
Tera Type: Fighting
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Behemoth Blade
- Play Rough
- Wild Charge
- Close Combat
It is one of the best offensive mons in the tier and is very naturally fast. I tried having swords dance over play rough or wild charge before but I feel it already has enough attack, especially the first time it switches in. Wild charge is super good for flying types and primal kypgre. Tera fighting close combat is just a strong move.
Yveltal @ Darkinium Z
Ability: Dark Aura
Shiny: Yes
Tera Type: Poison
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Defog
- Dark Pulse
- Oblivion Wing
- Foul Play
It is an offensive and speedy yveltal. Z-Dark pulse ko's almost all primal groudon and it can switch into it well. It works as a great lead option with the z move too. Oblivion wing helps it 1v1 some things for a bit and is its only form of recovery. Defog is general utility. Foul play is good becuse it can hit it on a boosted threat and allows me to revenge kill with lucario or flutter mane.
Arceus-Water @ Splash Plate
Ability: Multitype
Tera Type: Water
EVs: 248 HP / 136 Def / 124 Spe
Bold Nature
IVs: 0 Atk
- Judgment
- Ice Beam
- Recover
- Calm Mind
Bulky water who can come in and deal damage. Needs some calm minds to do major damage and has poor matchup with both primals. Ice Water is good coverage but does feel like it hits hard. However, it takes hits extremly well and can attack multiple times. It is also pretty fast vs some slower mons. Mainly here to help with dragons and chip things down for cleanup.
Groudon-Primal @ Red Orb
Ability: Desolate Land
Shiny: Yes
Tera Type: Ground
EVs: 252 HP / 4 Atk / 252 Def
Lax Nature
- Stealth Rock
- Precipice Blades
- Overheat
- Toxic
This primal is a blanket check for all phisical threats. Can set rocks easily. Precipice blades without investments hits super hard and if it's super effective, guarenteed ko. Overheat is just generally good and I feel like it gets better results than rock coverage. Toxic helps hits things on the switch.
Flutter Mane @ Booster Energy
Ability: Protosynthesis
Tera Type: Fairy
EVs: 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Calm Mind
- Moonblast
- Shadow Ball
- Thunderbolt
Booster speed flutter mane is really good revenge killer. Immune to e-speed helps vs a chipped e-killer arceus. Calm mind powers up the moon-blast. Tera fairy does so too. Thunderbolt is just coverage for ho-oh and kyogre. I almost never click shadow ball but it is STAB.
Let me know what changes I should make to make the team better.
Lucario-Mega @ Lucarionite
Ability: Justified
Tera Type: Normal
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Close Combat
- Meteor Mash
- Swords Dance
- Extreme Speed
I picked this mainly because I like it. Extreme speed is good priority and helps pick off weak mons reliably. It is a good speed tier in order to naturally outspeed a lot of bulky mons but is super frail. I have been thinking of replacing swords dance since I almost never use it in battle and I get the attack boost from meteor mash more often. Close combat is the highest damaging move.
Zacian-Crowned @ Rusted Sword
Ability: Intrepid Sword
Tera Type: Fighting
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Behemoth Blade
- Play Rough
- Wild Charge
- Close Combat
It is one of the best offensive mons in the tier and is very naturally fast. I tried having swords dance over play rough or wild charge before but I feel it already has enough attack, especially the first time it switches in. Wild charge is super good for flying types and primal kypgre. Tera fighting close combat is just a strong move.
Yveltal @ Darkinium Z
Ability: Dark Aura
Shiny: Yes
Tera Type: Poison
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Defog
- Dark Pulse
- Oblivion Wing
- Foul Play
It is an offensive and speedy yveltal. Z-Dark pulse ko's almost all primal groudon and it can switch into it well. It works as a great lead option with the z move too. Oblivion wing helps it 1v1 some things for a bit and is its only form of recovery. Defog is general utility. Foul play is good becuse it can hit it on a boosted threat and allows me to revenge kill with lucario or flutter mane.
Arceus-Water @ Splash Plate
Ability: Multitype
Tera Type: Water
EVs: 248 HP / 136 Def / 124 Spe
Bold Nature
IVs: 0 Atk
- Judgment
- Ice Beam
- Recover
- Calm Mind
Bulky water who can come in and deal damage. Needs some calm minds to do major damage and has poor matchup with both primals. Ice Water is good coverage but does feel like it hits hard. However, it takes hits extremly well and can attack multiple times. It is also pretty fast vs some slower mons. Mainly here to help with dragons and chip things down for cleanup.
Groudon-Primal @ Red Orb
Ability: Desolate Land
Shiny: Yes
Tera Type: Ground
EVs: 252 HP / 4 Atk / 252 Def
Lax Nature
- Stealth Rock
- Precipice Blades
- Overheat
- Toxic
This primal is a blanket check for all phisical threats. Can set rocks easily. Precipice blades without investments hits super hard and if it's super effective, guarenteed ko. Overheat is just generally good and I feel like it gets better results than rock coverage. Toxic helps hits things on the switch.
Flutter Mane @ Booster Energy
Ability: Protosynthesis
Tera Type: Fairy
EVs: 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Calm Mind
- Moonblast
- Shadow Ball
- Thunderbolt
Booster speed flutter mane is really good revenge killer. Immune to e-speed helps vs a chipped e-killer arceus. Calm mind powers up the moon-blast. Tera fairy does so too. Thunderbolt is just coverage for ho-oh and kyogre. I almost never click shadow ball but it is STAB.
Let me know what changes I should make to make the team better.
: Well, let's just say this is almost entirely outclassed by Marshadow, not only it's way faster, its practically unresisted STAB combination means that it is way harder to immediately wall, and it can even compress priority with a Technician-boosted Shadow Sneak. The only reason you'd want to use Mega Lucario is for the raw damage output of its Fighting STAB without a choice lock, but in a metagame where the top walls handle that comfortably, it doesn't seem justificable here. In particular I'd recommend the 
: Swords Dance is more valuable than what you'd think, it allows Zacian-C to OHKO Ho-Oh with Wild Charge over being rendered passive by Whirlwind (thus removing Intrepid Sword), Behemoth Blade also hits nothing relevant, so you can place it over that move. In a way you can never have too much Attack, as being able to just OHKO all remaining Pokemon by setting up on a forced switch is a key thing to sweep and end a game.
: Z-Dark Pulse is strong on paper, sure, but offensive Z-Moves generally shine on a setup sweeper, otherwise they can't really snowball to overwhelm a check and then threaten with a sweep, it can't even OHKO Ho-Oh, the main special wall of the metagame, so it does quite little against balance teams, which have been raising in viability ever since Xerneas got banned. Foul Play on a offensive stat spread is also rather suboptimal as you can't consistently expect Yveltal to avoid a 2HKO to then hit a foe with that move, instead being limited to a speed tier advantage, which means it just gets KOed by stuff like Dragon Dance Necrozma-DM or Arceus-Ground. I'd recommend to just use the
: Looks good as is!
: That set is suboptimal for a variety of reasons, for one the speed tier of uninvested Primal Groudon hits nothing of note, so you can afford just running a Relaxed (+Def, -Spe) nature, secondly that much physical bulk leaves it vulnerable on the special side, for instance you only need 48 EVs with a +Def nature to avoid a OHKO from a +2 Tera Blast Ground from Zacian-C, so any further bulk is rather situational, I'd recommend the spread listed for the 
: Besides being a very niche option for reasons including having very minimal defensive utility, lacking the moveslots to be able to play around multiple metagame staples, and being easy to force out by Primal Groudon and Ho-Oh (in the latter case literally out of Whirlwind), this becomes usually difficult to justify, plus the proposed previous swap for Marshadow over Mega Lucario compresses the intent of this Pokemon well anyways. The team is looking weak to special breakers like Deoxys-Attack and Meteor Beam Eternatus, so I'd suggest over it 
Marshadow can OHKO Necrozma with Tera Ghost Shadow Sneak, or without using tera with stealth rock or other chip damage. It can also deal a surprising amount of damage neutrally with a Bulk Up. Marshadow can also do a lot of things that Flutter Mane can do as well, like getting rid of cheesy ladder threats like Arceus-N and Basculegion-M.
Swords Dance is also good, so I included that over BBlade. Zacian is great speed control and can beat things like Yveltal and Arceus-N. Something you might want to consider is Tera Blast Ground, which hits more common threats super effectively (NDM, Eternatus, and Groudon-P) than CC.
ZDark isn't the best option (I think LO is generally better, Yveltal actually clicks Dark Pulse a lot) and I think Yveltal would prefer HDB because it's your hazard removal but is weak to stealth rocks. I opted for the defensive set, since as Bob said, Foul Play is less efficient on an offensive set. Yveltal isn't the fastest mon anyways, so it wants that extra defense if it's using Foul Play. Toxic/Taunt are there to prevent things from coming in and setting up on it and also for things that Foul Play doesn't deal much damage to.
Not much to say here but I did change the Tera type to Grass because this is a defensive set.
Your set here isn't that great. The moves are fine, but the EV spread is lacking. You don't want to take too much damage from something like a Kyogre-P Ice Beam or get OHKO'd by a Yveltal Tera Dark Dark Pulse. I changed the nature to -Spe and invested in some Spd. If you want more speed, it's an option to take off Overheat for Stone Edge and use a -Spa nature.
For the last member of the team, you could definitely go with NDM (maybe even Zygarde?) but I suggest Meteor Beam Eternatus. You only have one (and a half counting Groudon-P) special attacker (Arceus-W) so you want something with a high special attack. Meteor Beam Eternatus can cover Ho-Oh, Giratina (Altered and Origin), Palkia, survives a Zacian-C Play Rough and OHKOs with +1 Fire Blast. Dynamax Cannon on Eternatus also helps against Zygarde, although Tera Water is always a problem. Defensive Eternatus is also an option to wall Yveltal with Recover and survive a +1 Zekrom Dragon Claw but I used the Meteor Beam set because it helps with the Ho-Oh matchup. I think you need something else that can 1/2HKO Ho-Oh but doesn't risk taking a ton of damage or getting burned (i.e. Zacian-C) since you aren't using Stone Edge on Groudon-P.