I think the Fire type is the most difficult starting type for a Monotype Run.  Fire Pokemon can be relatively rare, are not that diverse, and most crucial of all, are again and again exposed to their weaknesses.  However, unlike the Grass type, Fire is an amazing attacking type and you can hit a lot of Pokemon with super effective damage!  You also get some great resistances to Ice and Fairy types!  Although there are some fun runs out there, be prepared to be exposed to at least one of your weaknesses.  Let’s take a look!

Monotype Run Rules

1. Only Pokémon of a certain type may be caught and trained.
2. You must catch the first Pokémon available of that type if your starter does not match that type (you’ll then have to discard that starter).
3. You may train a Pokémon that evolves to said type as long as you do it ASAP.
4. No trading allowed.
5. Mega Pokémon count as long as you Mega Evolve them as soon as they appear on the battlefield.
6. Only Pokémon caught before Elite Four are counted.

Best Games

By far the best game in the series for a Fire run is UltraSun followed by Sun.  No other games except Pokemon Black (and that’s barely because you get Reshiram at the end of the game) neutralizes your weaknesses.  But of course, you need more than just coverage for a great team and these games have it!  By Generation VII, Torkoal gets the Drought ability which temporarily increases Fire attacks, decreases Water attacks, and makes Solarbeam a one-turn attack.  That’s fantastic!  You also get a variety of strong Pokemon such as other Fire starters via the Island Scan feature!  UltraSun wins out over Sun just for having more Pokemon like Volcarona, Charizard, and Houndoom.

There are certainly other games that are great for Fire teams but you won’t have complete coverage against your weaknesses.  Basically any of the games from Generation VI and on I would put on this list as there’s a lot of good diversity and strength in these teams.

Worst Games

I would say Pokemon Yellow is the worst of the bunch but it IS a close call.  Generation I, in general, is very poor for many Monotype Teams but Fire seems particularly crappy because of its homogeneity and restrictive movepool.  More importantly, since Pikachu is your starter (and not a Fire type like virtually every other game) then your first Fire Pokemon is a Charmander right before the second gym so that will take awhile  (then of course you have to face Misty so good luck!!).

There are many memes regarding Diamond and Pearl’s incredibly awful Fire Pokemon and it’s frankly deserved.  You get Chimchar (good start), then a Rapidash (alright…) and that’s it.  Just two Pokemon!!!  Seriously??  At least Infernape is excellent so you might as well do a Single Pokemon challenge and just grind your starter.  Don’t even bother with that Rapidash…

Fire Teams in Pokemon Games

Red, Blue, Yellow, FRLG
Ideal Team: Charizard, Arcanine (Red, FireRed, Yellow)/Ninetales (Blue, LeafGreen, Yellow), Magmar (Blue, LeafGreen), Flareon, Rapidash, Moltres
First Pokémon: Charmander via starter or via Route 24 after Mt. Moon in Yellow.
Covers weaknesses? No, Rock or Water not neutralized.

Gold, Silver, Crystal, HGSS
Ideal Team: Typhlosion, Magmar, Ninetales (S, SS)/Arcanine (G, C, HG), Flareon/Magcargo (HGSS), Rapidash, Ho-Oh (G, HG), Entei
First Pokémon: Cyndaquil via starter
Covers Weaknesses? No; Rock, Water, and Ground (besides Ho-Oh) are not covered
Note: In HGSS, you can get a Slugma Egg from Primo in Violet City center.  You’ll need to give him a phrase which is dependent on your Trainer ID which you can receive here.

Ruby, Sapphire, Emerald, ORAS
Ideal Team: Blaziken, Camerupt, Magcargo, Ninetales, Volcarona (ORAS), Arcanine (ORAS)
Optional: Torkoal, Flareon (ORAS), Magmar (ORAS), Rapidash (ORAS), Ninetales (ORAS), Darmanitan (ORAS), Primal Groudon (OR)
First Pokémon: Torchic via starter
Covers Weaknesses? No, Water and Ground not neutralized.  In OmegaRuby, Water can be taken care of due to Primal Groudon’s Ability.

Diamond, Pearl, Platinum
Ideal Team: Infernape, Rapidash, Flareon (Platinum), Houndoom (Platinum), Magmar (Platinum)
First Pokémon: Chimchar via starter
Covers Weaknesses? No, Ground and Water not neutralized

Black, White, B2W2
BW Ideal Team: Emboar, Darmanitan, Chandelure, Volcarona, Heatmor, Reshiram (B)
Optional: Simisear
First Pokémon: Tepig via starter
Cover weaknesses? Only in Pokemon Black and even then it happens at the very end when you get Reshiram.  Otherwise you’re exposed to Water moves in both versions.

B2W2 Ideal Team: Emboar, Arcanine, Magmar (B2)/Camerupt (W2), Darmanitan, Volcarona, Chandelure
Optional: Flareon, Simisear, Darmanitan
First Pokémon: Tepig via starter
Cover weaknesses? No, Water is not neutralized

XY
Ideal Team: Delphox, Talonflame, Charizard, Pyroar, Houndoom (X), Chandelure
Optional: Simisear, Flareon, Magcargo, Torkoal, Heat Rotom
First Pokémon: Fennekin via Starter
Weaknesses Covered? No, in Pokemon X, Rock is not neutralized.  In Pokemon Y, Water and Rock are not neutralized.

Sun, Moon, USUM
SM Ideal Team: Incineroar, Arcanine/Turtonator (Sun), Talonflame, Torkoal, Marowak, Emboar (scan)
Optional: Flareon, Chandelure (scan), Salazzle, Magmar, Typhlosion (scan), Oricorio
First Pokémon: Litten via Starter
Cover weaknesses? Only in Sun thanks to Turtonator. In Moon, Water is not neutralized. However, you can cut back on Water’s strength if you train a Torkoal which now can learn the ability Drought.

USUM Ideal Team: Incineroar, Arcanine/Turtonator (US), Charizard (scan), Torkoal, Marowak, Blaziken (scan)
Optional: Flareon, Chandelure (scan), Pyroar, Talonflame, Infernape (scan), Delphox (scan), Salazzle, Volcarona, Magmar, Houndoom (US), Oricorio
First Pokémon: Litten via Starter
Cover weaknesses? Only in Ultra Sun thanks to Turtonator. In Ultra Moon, Water is not neutralized. However, you can cut back on Water’s strength if you train a Torkoal which now can learn the ability Drought.

Sword, Shield
Ideal Team: Cinderace, Torkoal, Centiskorch, Coalossal, Heat Rotom, Turtonator (Sword)/Arcanine
Optional: Flareon, Ninetales, Salazzle, Chandelure, Heatmor, technically Darmanitan in Zen Mode
First Pokémon: Scorbunny via Starter
Weaknesses Covered? Sadly no, Shield is weak to Water and Rock.  Sword is weak to just Rock thanks to Turtonator.

MVP (Most Valuable Pokemon)

Fire Starters
Of course!  Your starter!  Really, what saves the Fire type from abysmal Monotype Runs is Charmander and friends.  They make a HUGE difference in your Monotype Run.  And the best part is, starters rank among the strongest for their types and are packed with awesome moves, great type combos, and wonderful stats.  You won’t be disappointed with them.

The Fire starter has more dual types in its line up than the other two starters.  This automatically improves your diversity and, in some cases, eliminate a weakness.  Of course, you have the three Fighters, Blaziken, Infernape, and Emboar.  They neutralize Rock attacks AND the Fire+Fighting STAB is one of the best dual offensive moves in the game (also Emboar can also learn Scald which is nice against your Ground and Rock foes!).  The classic Charizard gives you immunity to Ground attacks and gives you some nice Dragon moves and if you have Charizard X you can have a soft counter to Water foes.  Delphox and Incineroar, while they don’t give you any resistances, still give you move diversity along with unique, level-up attacks.  Typhlosion and Cinderace unfortunately get overshadowed by these starters but at least the Typhlosion line is adorable!

Also, don’t forget to catch other Fire starters in Generation VII!  The Island Scan feature is amazing and is one of the reasons why you can cover your weaknesses thanks to the Fighters.
Available in: Every game.  Only in Pokemon Yellow and the Let’s Go games do you not get a Fire starter but Charmander still appears in those games.

Arcanine
The most perfect boi is the most perfect team member.  I kid you not, Arcanine is the strongest non-Mega, non-Legendary Fire Pokemon.  Like, what?!?!  Seriously?  That’s awesome!  And Arcanine is one of the most common Fire Pokemon in the entire series!  Super spectacular.  Arcanine also gets progressively better and by Generation V really becomes a must-have as it can learn Outrage and Wild Charge and a few other cool moves.  Arcanine’s biggest weakness is its single typing which doesn’t give you a diversity boost but has far as downsides go that’s not bad!
Available in: Red, Yellow, Gold, Crystal, FireRed, HeartGold, B2W2, ORAS, SM, USUM, SWSH 

Fire Bugs
Volcarona and Centiskorch I think are great members that will add a nice flavor to your overall vanilla team.  First, they offer a soft counter to Ground Pokemon and Centiskorch can learn a lot of anti…anti-Fire Pokemon moves like Powerwhip, Scald, and Thunder Fang.  Volcarona is also among the strongest Fire Pokemon and pair that with Quiver Dance and Fiery Dance means you can easily set up and sweep opposing teams.  Volcarona is pretty common too and you can get a fully evolved one in B2W2 which is fantastic.  Of course, there’s a foreboding 4x Rock weakness which is tricky when you fight Ground foes.  Also, Volcarona evolves very late so you’ll be stuck with Larvesta for quite awhile in many of the games…
Available in: Volcarona in BW, B2W2, ORAS, USUM and Centiskorch in SWSH

Chandelure
The wonderful Chandelure has among the highest Special Attack stats out of all Fire Pokemon with a whopping 145!  Even STAB attacks that deal neutral damage will still deal a great deal of pain to your opponents.  You can also teach it Energy Ball so immediately you have a counter to all your Fire foes.  Chandelure’s Ghost characteristics come through and give it a lot of great status moves like Calm Mind, Will-O-Wisp, Hex, and Confuse Ray.  Chandelure is also surprisingly common and you can find it in many games from Pokemon BW onwards.  The biggest trade off is Chandelure is a bit fragile and not that fast so it may faint on you before you can even do anything.
Available in: BW, B2W2, XY, SM, USUM, SWSH

Torkoal
Some Pokemon are here through a combination of their stats, their moves, or their availability; Torkoal is here solely for its ability.  Starting in Generation VII on, Torkoal can learn the Drought Ability which gives it automatic Sunny Day status for five turns once it enter the battle.  This means you don’t have to set up a Sunny Day move and instead go right into using boosted Fire attacks and an instant Solarbeam on your opponents!  Also, Torkoal has one of the highest defense stats (140) out of any Fire Pokemon and considering Fire’s defense is among the weakest for the types that’s pretty nice.  Torkoal can also learn strong Ground and Rock moves too which is a plus.  Still, not much else to talk about Torkoal AND it’s pretty slow so just keep that in mind as you’re training with one.
Available in (with Drought): SM, USUM, SWSH

Turtonator
Any Swimmer you will face will be a constant struggle of agony and hardship.  There are frankly very few Fire Pokemon that neutralize their Water weakness (as of Generation VII over 3/4 of the Fire teams are exposed to Water attacks).  Charizard X is going to be one of them but given its rarity you might be better up with Turtonator.  On the whole, there’s not a lot going for Turtonator besides its Dragon typing.  It has okay stats, it’s only in three games so far, and it has an alright ability (Shell Armor which protects against Critical Hits).  Still, it’s better than nothing and you can at least teach it some good Rock and Ground moves.
Available in: Sun, UltraSun, Sword