While this question isn't clear weather it's about competitive or in game team, I will give a shot at in game team, seeing as I'm not really qualified to give advice on competitive hahaha!
In Game Team
1. Type Advantages and Disadvantages
So first, we need to think about weaknesses and strengths. Fire is weak to Rock, Ground, and Water. Ice is weak to Fire, Fighting, Rock, and Steel. Straight away, we can see that Fire is good against Steel types, and Ice is good against Ground types, and if you can get something with Freeze Dry, it covers Water types as well. Another thing we can see, is that Rock cripples your team. If you are doing a Pokemon Sword Challenge Run, beware of Gordie!! You need a cover to Rock types, so I suggest using Lapras. It's got great Water STAB that your team really needs.
2. The Pokemon
Okay, so I'm gonna assume that you want it balanced, otherwise "it's a Fire Monotype run with like a Lapras because it's really an Ice/Fire run". No. Juuuuuuuuust no. Let's make it 3-3, but if you don't like it you can change it.
Fire Types: These are Fire types that are great to have on the team. Most aren't that hard to get, but they are great additions for the team!
Cinderace. It's a fast Physical sweeper, and with access to loads of different moves and a wide coverage, it is great as a Fire type. Better, you can get it at Postwick as your starter!
Centiskorch. You can get a Sizzlipede on Route 3, after the Gym Challenge opening ceremony, and it has a great potential and a wide Physical moveset. I would get this guy on a Fire/Ice playthrough :)
Arcanine. If you are playing Shield, you can find some Growlithes on Route 3. If you're playing Sword then you can get them in the Wild Area in harsh sunlight. Arcanine has a great Physical learnset, but an underrated Special one too. If you use all these Fire types, they're all Physical. Arcanine, however, can benefit from using Sunny Day, combined with Flamethrower, Solar Beam, and Dragon Pulse, which covers all the weaknesses +Dragon (which resists Fire/Grass)! If there's another Sunny Day Pokemon, you can always swap Sunny Day on Arcanine to something like Scorching Sands or Snarl. But then again, a Physical one is good too!
Torkoal. Good learnset as well as the amazing ability Drought, setting up for your Fire types. It can be obtained in Warm-Up Tunnel if you have the DLC, and in the Wild Area too, but only at a higher level. I recomend using the DLC to get one, but if you don't have the DLC, just wait til Torkoal is easier to get.
Pokemon To Consider ( some if you have the DLC):
1. Talonflame. A Flying type is always good for a playthrough team, with a great Speed stat, it's not one to be looked over.
- Incineroar. If you pick Scorbunny and manage to collect 100 Digletts from the Diglett guy, he will award you with a Hidden Ability Litten. It might take a while, but there are plenty of online guides with locations for all the Digletts. Incineroar has a great Dark typing, as well as amazing Physical attack and somewhat good Defenses. A wide moveset as wekk, with things like Earthquake, Darkest Lariat, Outrage, Iron Head, and Thunder/Fire Punch.
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Now onto the Ice types! I have 4 of these that I think are great, and can be obtained without much hassle.
Abomasnow. You can obtain a Snover in the WIld Area before the first Gym. It has quite a late evolution, but that shouldn't be too much of a problem, and it's definately worth the wait, as Abomasnow has Grass STAB, which is Super Effective against all of Fire's weaknesses. Seed Bomb, Wood Hammer, Ice Punch, Earthquake, Outrage, and Rock Tomb are just some of the great moves it can learn.
Weavile. It has an insane Speed/Attack stat, and is really good Physical learnset, as well as having access to Sneasel in the Wild Area early on in the game. Psycho Cut, Shadow Claw, Night Slash, Poison Jab, X-Scissor, and Swords Dance are all very viable. The only drawback is that it isn't easy to find a Razor Claw, but it should be okay!
Arctozolt. He's one chilly boi. I love the Galarian fossils ^^. A bit of mashing the A button near the Skill brother of the Digging Duo will get the fossils with relative ease, although it all depends on luck. You can get one after beating Kabu, which is great. Although it's somewhat lacking in Speed, that's not a hard problem to fix with some small EV training. Bolt Beak, Body Slam, Stone Edge, Icicle Crash/Ice Fang, and Rock Slide/Blast are all good. Thunder Fang and Wild Charge are both great too, until you get Bolt Beak.
Lapras. It's a Water type, which is great for the team. It has equal Physical and Special stats, but I'd make it Special. Freeze Dry, Thunderbolt, and Psychic are all great, and Weather Ball can also be very good if you have Sunny Day set up. If you can breed it with a Primarina, however, it gets Sparkling Aria as an egg move. Awesome :D
Another Pokemon of note, is the famous Galarian Darmanitan. Ot is the Only Fire/Ice type to exist so far, but only in Zen Mode. Although Galarian Darmanitan's Defenses are lacking, it has an Attack stat of 140. Wow. It has a great Physical learnset, and if you can get a HA one through Max Raids or otherwise, then at 50% HP, it will turn into Zen Mode, the only Fire/Ice type in the games. It's a good Pokemon for the team.
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Okay! So I have mentioned all of the Pokemon that I think are good for a Fire/Ice challenge run. Let's Go over everything. We have:
Fire:
- Cinderace
- Centiskorch
- Arcanine
- Torkoal
- Talonflame
- Incineroar
Ice:
- Abomasnow
- Weavile
- Arctozolt
- Lapras
- Galarian Darmanitan.
Hope I helped, I put a lot of effort into this! :D Good luck with the team, too!
