This team could be definitely good if you were to bring it in tournament battles, but it's not the best to use in ladder as you lack a Stealth Rock setter which is extremely vital. Scarf Calyrex-S owes no place in a HO, and if you're facing problems vs Kyogre (which you shouldn't) then replace some of the mons.
Grimmsnarl @ Light Clay
Ability: Prankster
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Reflect
- Light Screen
- Play Rough
- Taunt
You don't need both Thunder Wave and Taunt in same set, else Yveltal just walls the hell of you and can just easily Defog away your screens. Play Rough is chosen to deal some good damage to it, notably over Spirit Break so that you out-damage Roost recovery.
Calcs
- 0 Atk Grimmsnarl Spirit Break vs. 248 HP / 8 Def Yveltal: 198-234 (43.5 - 51.4%) -- 5.9% chance to 2HKO
- 0 Atk Grimmsnarl Play Rough vs. 248 HP / 8 Def Yveltal: 236-278 (51.8 - 61%) -- guaranteed 2HKO
Groudon --> Excadrill
Excadrill @ Focus Sash
Ability: Mold Breaker
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Stealth Rock
- Rapid Spin
- Toxic / Rock Tomb
- Earthquake
As I told before, you definitely need Stealth Rock if you're laddering, so as to not get trolled on by 6 random Focus Sash users (Marshadow noises). As such, we're going Excadrill which can set Stealth Rock of its own while keeping those away from your side as well. So this is a pretty a simple set, with this being the rocker of our team as well as spinner. Toxic allows it to cripple Lunala and Yveltal, which can just Defog your rocks. Earthquake is STAB and annoys NDM.
> Necrozma-Dusk-Mane @ Weakness Policy
Ability: Prism Armor
EVs: 80 HP / 252 Atk / 176 Spe
Adamant Nature
- Dragon Dance
- Sunsteel Strike
- Earthquake
- Stone Edge
This set is fine, though I had say you could opt for a bulkier spread so that you could tank more hits paired with Screens. This is the standard EV spread for offensive DD which allows it to outspeed Scarf ogre, Calyrex-S and pretty much every non-scarfer except Regieleki.
Zacian-Crowned --> Zekrom
Zekrom @ Leftovers / Life Orb / Weakness Policy
Ability: Teravolt
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Dragon Dance
- Bolt Strike
- Dragon Claw
- Substitute / Roost
If you're running Zacian-C in a Screens HO, then that's a big mistake. Since it can't hold an item like Weakness Policy to become much more threatening, it's not doing fireworks and stuff as it can't even beat Defensive NDM if it Dynamaxes whether Reflect is up or not. Unlike the former, this Pokemon abuses screens like Hail, as it forces you to hit with Super Effective move activating Weakness Policy. Substitute just dodges status and is better vs Lunala, however Roost is also good vs non EQ NDM variants.
Rather is self-explanatory who needs to know the reasoning of the most simple set :)
Yveltal is golden.
Calyrex-Shadow @ Life Orb / Focus Sash
Ability: As One (Spectrier)
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Astral Barrage
- Nasty Plot
- Substitute / Psyshock
- Draining Kiss
The Scarf Calyrex-S set you're running...is absolutely terrible. Scarf runs Astral Barrage / Psyshock / Trick / Aromatherapy, not full coverage! Even though, it's a huge momentum lose for Screens to not run a Nasty Plot Calyrex-S, so I suggest that. Life Orb gives power, notably 2HKOing Ho-Oh with Astral Barrage and Yveltal +2 with Max Starfall. Alternatively, Focus Sash lives a hit and can take Scarf Caly-S, Kyogre, etc. by surprise. Nasty Plot allows you to just OHKO / 2HKO everything. Substitute with Life Orb and Psyshock with Focus Sash.
Final Team
Hope this helps!