Ok, I made some HUGE changes to the team, and I'd love some feedback on it now.
Gardevoir-Mega @ Gardevoirite
Ability: Pixilate
EVs: 240 HP / 240 Def / 28 SpA
Calm Nature
- Calm Mind
- Hyper Voice
- Shadow Ball
- Psychic
Skarmory @ Leftovers
Ability: Sturdy
EVs: 252 HP / 76 Def / 180 SpD
Impish Nature
- Stealth Rock
- Spikes
- Whirlwind
- Roost
Excadrill @ Assault Vest
Ability: Mold Breaker
EVs: 252 HP / 168 Atk / 88 SpD
Adamant Nature
- Rapid Spin
- Earthquake
- Iron Head
- Rock Slide
Landorus-Therian (M) @ Choice Scarf
Ability: Intimidate
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Earthquake
- Stone Edge
- Superpower
- U-turn
Kingdra @ Scope Lens
Ability: Sniper
EVs: 12 HP / 252 SpA / 244 Spe
Modest Nature
IVs: 30 Atk / 30 SpA / 30 Spe
- Focus Energy
- Draco Meteor
- Surf
- Agility
Breloom @ Toxic Orb
Ability: Poison Heal
EVs: 252 HP / 56 Atk / 200 Spe
Jolly Nature
- Substitute
- Spore
- Focus Punch
- Seed Bomb
I don't think that Mega Gardevoir should use a Calm nature to bolster up its special bulk. It's already fine uninvested. You're also not taking advantage of the 100 Speed Gardevoir has in Mega form. It's still physically frail, even with investment. However, the below set has minimal defensive investment to let Mega Gardevoir survive 2 Psyshocks from Latios:
-Psyshock
-Hyper Voice
-Focus Blast/Hidden Power (Ground)
-Taunt/Will-o-Wisp/Calm Mind
Nature: Timid/Modest
EVs: 24 Def/232 SAtk/252 Spe
Item: Gardevoirite
Ability: Trace
Skarmory is horrible Taunt bait if it opts for both of its hazards. Just stick to one of them and you're good. Since this is a Showdown! team, use this Skarmory instead:
-Roost
-Defog
-Whirlwind/Taunt/Brave Bird
-Toxic/Stealth Rock/Counter/Brave Bird
Nature: Bold/Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
Assault Vest Excadrill should use 120 HP/136 Atk/252 SDef for optimum special bulk.
Use this set on Therian Landorus:
-Earthquake
-Stone Edge
-U-turn
-Knock Off/Superpower
Nature: Adamant
EVs: 140 HP/156 Atk/212 Spe
Item: Choice Scarf
The EV spread has enough Speed to let Therian Landorus outspeed Mega Manectric, while the Attack and HP EVs let it still hit hard and make the most out of Intimidate.
And if you can't fit Stealth Rock on to Skarmory, you might as well put it on Therian Landorus:
-Stealth Rock
-Earthquake
-U-turn
-Stone Edge/Knock Off
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers/Rocky Helmet
I don't think that Poison Heal Breloom is viable anymore. It's walled by so many things, including Mega Venusaur and Amoonguss, that it's not even funny. The Technician set is the way to go this time. I'd rather have a move that hits both Pokemon neutrally than be walled entirely by them, IMO. However, Gardevoir can handle the aforementioned Pokemon, giving Breloom some breathing room:
-Bullet Seed
-Mach Punch
-Rock Tomb
-Spore
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Focus Sash
Ability: Technician