Gen 6 I Just need some advice with this new team that i am making

[Majora's Kirbo]

So basically, i made a Gardevoir Team a bit time ago : https://www.smogon.com/forums/threads/rate-my-mgarde-sd-weavile-balance.3770372/

And now, i revisited this team in something that i really like, with Volcarona:

Gardevoir (M) @ Gardevoirite
Ability: Trace
Shiny: Yes
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Hyper Voice
- Psyshock
- Focus Blast
- Will-O-Wisp

Hélios (Volcarona) (M) @ Lum Berry
Ability: Flame Body
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Quiver Dance
- Flamethrower
- Bug Buzz
- Giga Drain

Thundurus (M) @ Rocky Helmet
Ability: Prankster
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk / 30 Def
- Thunder Wave
- Hidden Power [Ice]
- Focus Blast
- Volt Switch

Keldeo @ Leftovers
Ability: Justified
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Substitute
- Calm Mind
- Scald
- Secret Sword

Excadrill (M) @ Choice Scarf
Ability: Mold Breaker
Shiny: Yes
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Rock Slide
- Rapid Spin
- Earthquake
- Iron Head

Jirachi @ Leftovers
Ability: Serene Grace
Shiny: Yes
EVs: 252 HP / 224 SpD / 32 Spe
Careful Nature
- Iron Head
- Stealth Rock
- U-turn
- Fire Punch


Can someone help me, if i made some mistake or anything else...!

 

[Paprikaflow]

You made no mistake, but there are some things that I think would generally benefit your team.

For me the main issue is that your team is relatively unable to keep Stealth Rock off the field, yet Volcarona is very vulnerable to it.
The second big issue I see is, how weak you are to Water-types like Keldeo or Volcanion.

I would consider changing Jirachi in total, as it is too slow and doesn't check enough. Instead you could think about using a fast Landorus-T with Stealth Rock and Imprison (to prevent opposing Stealth Rock).
To fix the issue with the Water-type-weakness your could think about replacing Excadrill with another Pokemon able to remove Stealth Rock, like Latias. This on the other hand would make the match up against Clefable worse and would mean that you do not have a Steel-type on the team. So there remain some issues.
If you do not want to make any of these changes, then you could still think about making Volcarona Sitrus Berry. This mitigates the Stealth Rock damage for Volcarona.

Generally you can use another Mega-Gardevoir spread:
Gardevoir @ Gardevoirite
Ability: Trace
EVs: 16 HP / 8 Def / 232 SpA / 252 Spe
Timid Nature
- Hyper Voice
- Psyshock
- Focus Blast
- Will-O-Wisp

as explained here: https://www.smogon.com/dex/xy/pokemon/gardevoir/