my team is good but it is starting to lose whats happening pls find any flaws

Question

also i will not use chansey because i prefer more hp than defense

Blissey @ Leftovers
Ability: Natural Cure
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Aromatherapy
- Soft-Boiled
- Toxic
- Seismic Toss

Zapdos @ Leftovers
Ability: Pressure
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Heat Wave
- Roost
- Discharge
- Defog

Tapu Lele @ Psychium Z
Ability: Psychic Surge
Shiny: Yes
EVs: 248 HP / 252 SpA / 8 SpD
Modest Nature
IVs: 0 Atk
- Psyshock
- Moonblast
- Calm Mind
- Shadow Ball

Greninja-Ash @ Choice Specs
Ability: Battle Bond
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Water Shuriken
- Dark Pulse
- Hydro Pump
- Ice Beam

Medicham-Mega @ Medichamite
Ability: Pure Power
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Fake Out
- High Jump Kick
- Ice Punch
- Zen Headbutt

Landorus-Therian (M) @ Assault Vest
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Knock Off
- Earthquake
- Explosion
- Return

Comments

What format?

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This is what I think:

Blissey only has 5 more base HP and can't use Eviolite, and therefore is sevearly outclassed by Chansey. :P
Mathematically, Defenses matter far more than HP in Pokémon:

252 Atk Kartana Sacred Sword vs. 248 HP / 252+ Def Eviolite Chansey: 334-394 (47.5 - 56%) -- 82.4% chance to 2HKO

252 Atk Kartana Sacred Sword vs. 252 HP / 252+ Def Blissey: 458-540 (64.1 - 75.6%) -- guaranteed 2HKO after Leftovers recovery

Tapu Lele is being used as a Calm Mind user but has no Speed or Recovery, the two things that typically make a Calm Mind user good. :P

You don't have a clear core; while this isn't required, it may improve your team. :P

I don't understand anything justifying using Assault Vest on Landorus-T. It only potentially stops a KO from other Landorus-T, which isn't really useful when you could be using a Focus Sash and have Stealth Rock Set or Suicide with Explosion guaranteed. I mean, look at how the Assault Vest holds up against other common Ice Attacks:

252 SpA Life Orb Teravolt Kyurem-Black Ice Beam vs. 0 HP / 8 SpD Assault Vest Landorus-Therian: 577-686 (180.8 - 215%) -- guaranteed OHKO

252 SpA Life Orb Latios Ice Beam vs. 0 HP / 8 SpD Assault Vest Landorus-Therian: 411-484 (128.8 - 151.7%) -- guaranteed OHKO

The difference is that, with an Assault Vest, Landorus-Therian is KOd, but with Focus Sash, Landorus-T KOs Latios, and cripples or also KOs Kyurem-Black. :P

Just a few things to consider. I hope I helped. :P

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In my opinion there seems to be no point of defog on zapdos especially with discharge give hime thunderbolt and maybe volt-switch

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Also sacrafice your speed on Landorus i know speed is pretty important in competitive but he is ground flying so he's pretty defense take switch spd and speed around

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Defog is for removing Entry Hazards and SpD Lando isn't great in any format. :P

Answer 1

Blissey only has 5 extra base HP over Chansey and generally lower bulk, making the HP gain negligible. Leftovers is nice but overall not worth it since Blissey can get OHKO'd by so many of the powerful physical attackers in OU while Chansey can take a hit or two from them while sponging special hits better.

Not necessarily something I'd change but I've seen some Zapdos sets run Thunder to smack Pokemon in the rain. Discharge is still better for its paralysis chance.

CM Lele with Psychium Z is... interesting. I suppose it's not terrible, since it can OHKO Chansey at +1, but as Stakatacool mentioned, Lele doesn't work because it's not fast enough and has no means of reliable recovery. Shadow Ball isn't the best coverage move either, and leaves Lele super susceptible to the numerous Steel-types in the tier. I'd swap Calm Mind for Taunt to cripple would-be walls, Shadow Ball for Focus Blast, and Psychium Z for Fightinium Z to obliterate usual checks like Magnezone and Heatran.
You might want max speed instead of max HP to ensure crucial outspeeds.

Assault Vest on Landorus-T doesn't really do much against the numerous heavy hitters in the tier and really only serves to prevent potential KOs that a Focus Sash set would do fine. Swap Return for Stealth Rock and you're pretty much good as a suicide lead.