Just so that we're on the same page, this is the Charizard moveset I'm talking about when I say Sunny Day Charizard.
Fire Dragon (Charizard) @ Petaya Berry
Ability: Blaze
EVs: 4 Atk / 252 SpA / 252 Spe Mild Nature IVs: 30 HP
- Fire Blast
- Focus Punch / Hidden Power [Grass] / Blast Burn
- Substitute
- Sunny Day
Now, here is list of good Pokemon to pair with Charizard, in descending order of synergy and viability.
1. Bulky Dugtrio
Dugtrio @ Choice Band Ability: Arena Trap
EVs: 40 HP / 252 Atk / 100 SpD / 116 Spe Jolly Nature
- Earthquake
- Hidden Power [Bug]
- Rock Slide
- Beat Up
Dugtrio traps Tyranitar so that Charizard can set up sun and spam Fire Blast freely. The speed EVs lets it outspeed +1 Adamant max speed Tyranitar. The HP and Sp.Def EVs allow it to reliably survive Blissey's Ice Beam. Beat Up is meant to hit Blissey harder than Earthquake and reliably 2HKO it. Dugtrio is the only Pokemon I would say is neccessary to run alongside Sunny Day Charizard.
Tremors (Dugtrio) @ Choice Band
Ability: Arena Trap
EVs: 40 HP / 144 Atk / 100 SpD / 224 Spe Jolly Nature
- Earthquake
- Hidden Power [Bug]
- Rock Slide
- Beat Up
This is alternative EV spread for teams weak to DD Tyranitar. It outspeeds all +1 speed Tyranitars, but loses out on many KOs due to reduced attack. Not reccomended unless you are utterly terrified of Tyranitar.
2. Offensive Celebi (CM Pass/ CM 3 attacks)
Pixie (Celebi) @ Leftovers
Ability: Natural Cure
EVs: 76 HP / 252 SpA / 180 Spe Timid Nature
- Calm Mind
- Giga Drain
- Hidden Power [Fire]
- Baton Pass / Psychic
Celebi can lure in and weaken Tyranitar for the benefit of Charizard. The two also cover each other's defensive weaknesses. It can also pass Calm Mind to make Charizard even more dangerous.
3. Offensive Suicune (Substitute + Calm Mind or CM 3 attacks)
Majesty (Suicune) @ Leftovers
Ability: Pressure
EVs: 252 HP / 104 SpA / 152 Spe Modest Nature
- Substitute
- Calm Mind
- Hydro Pump
- Ice Beam
Dugtrio traps Tyranitar and Charizard removes sand with Sunny Day. This can easily set the stage for Subsitute + Calm Mind Suicune to come in and sweep, especially against passive defensive teams. SubCM Suicune is an absolute menace when it isnt being constantly worn down with sand. It also gives your team more of a defensive backbone against physical threats like Salamence that can come in Charizard. Offensive Suicune is also good as a cleaner after Charizards weakens special walls.
4. Calm Mind Jirachi or Metagross
They cover Charizard's weakness to rock. Jirachi can lure in a weaken Tyranitar to enable Charizard, or Charizard can lure Tyranitar with a Focus Punch on the switch to enable Jirachi. Substitute + Calm Mind and Calm Mind 3 attacks Jirachi are both good Charizard team mates. Metagross can check Tyranitar and sweep late game with Agility or be a wallbreaker with Explosion.
5. Offensive Zapdos
It can bring in Charizard or Dugtrio against special walls like Jirachi, Celebi or Blissey with Baton Pass. You can also sacrifice it against Tyranitar T1 and then trap it with Dugtrio.
Lightning round of niche picks that synergise with Charizard
1. Porygon 2: Traps Dugtrio, provides a defensive backbone, downside of being slow and passive.
2. Kingdra: Benefits from removal of Tyranitar to sweep late game with Rain Dance.
3. Exeguttor: Has access to sleep powder, benefits from sun as it has Chlorophyll.
4. Lunatone: Gimmick sidegrade to Celebi that checks Salamence and has Hypnosis.
