This team has some issues in move and item choices, let me try a hand to see what I can fix.
Blissey (F) @ Utility Umbrella
Ability: Natural Cure
EVs: 4 HP / 252 Def / 252 SpD
Calm Nature
IVs: 0 Atk
- Seismic Toss
- Confide
- Aromatherapy / Stealth Rock
- Soft-Boiled
The set is alright, though Leftovers isn't the best choice here. Since it's what you're relying for checking Kyogre, it should have Utility Umbrella so to ignore the rain based damage and also allow you to live Specs Water Spouts much better.
- 252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 252+ SpD Blissey in Rain: 393-463 (60.2 - 71%) -- guaranteed 2HKO
With Utility Umbrella:
- 252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 252+ SpD Blissey: 262-309 (40.1 - 47.3%) -- guaranteed 3HKO
Also, the EV spread isn't the best here. Use 4 HP / 252 Def / 252 SpD, for real it's much better on Blissey.
Zygarde @ Leftovers
Ability: Power Construct
EVs: 204 HP / 248 Def / 56 Spe
Impish Nature
- Haze
- Thousand Waves
- Rest
- Toxic
That set you're having is real whack. Without recovery, this Pokemon can't really check anything. I don't see the point of "GlareSub" which is just a waste of moveslot. If you're using a defensive set, use the above Waves trapper set which is great at trapping Defensive mons such as Tangrowth and weakening them for the wallbreakers.
Alternatively, here's an offensive DD set:
Zygarde @ Leftovers
Ability: Power Construct
EVs: 16 HP / 224 Atk / 36 Def / 232 Spe
Impish Nature
- Thousand Arrows
- Substitute
- Toxic
- Dragon Dance
The EV spread allow it to live a +2 Groudon's Precipice Blade, also not let its Substitute broken by defensive Sunsteel Strike in complete forme so to set-up on its passivity. ToxicSub is great as it weakens its checks in Lunala, Xerneas and non Jungle Healing Zarude. Rest of the set is pretty self explanatory.
Yveltal @ Heavy-Duty Boots
Ability: Dark Aura
EVs: 16 HP / 240 SpD / 252 Spe
Jolly Nature
- Foul Play / Knock Off
- Sucker Punch
- Defog
- Roost
The set is alright, though you appreciate Max Spe over SpD here. Nowadays, this spread is becoming more common because of the Sticky Webs match-up. The Spe means you're much more having ease to Defog, while Foul Play just destroys Groudon and you outspeed it before Rock Polish.
Xerneas --> Lunala
Lunala @ Heavy-Duty Boots
Ability: Shadow Shield
EVs: 252 HP / 4 Def / 252 Spe
Timid Nature
- Ice Beam
- Roost
- Will-O-Wisp
- Roar
Since we already have Blissey which can provide Aromatherapy support, I doubt there's a reason for Xerneas to be used alongside it. Apart from that, Xerneas does not help vs the Groudon MU, and it's also getting 2HKOed by Life Orb Zekrom. Also, it's not having reliable recovery unlike Lunala. So, to fix the Groudon and Zekrom weakness this Pokemon will come into rescue.
Marshadow is alright and cool.
Kyogre ---> Calyrex-Shadow
Calyrex-Shadow @ Choice Scarf
Ability: As One (Spectrier)
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Trick
- Nasty Plot
- Astral Barrage
- Psyshock
Kyogre is a cool mon and destroys everything in its path, but what this team is missing out is speed control. None of the Pokemon here are faster than Eternatus or base 130s, so to fix them Calyrex-Shadow can prove helpful.
This is a Trick Nasty Plot set. As you trick your Scarf, its easier for it to set-up Nasty Plot vs defensive Pokemon and can be chipped much more easily. Psyshock is for the pink blobs.
Final Team
Hope this helps!
