In Gold and Silver, Magmar is available in Burned Tower's basement floor, with a 5% encounter rate during night and morning and a 10% encounter rate during the day. In Crystal, there's a chance of Magby hatching from the Odd Egg given on Route 34 by the Day Care couple, while wild Magmar can be found on Mt. Silver during the day.
An uncommon typing (especially in the Johto based games), together with the stats of a decently fast mixed sweeper, a decent special defense and a low physical bulk (watch out for physical attacks and moves alike!) are what makes Magmar an interesting Pokemon. However, its movepool is what makes it versatile, mainly thanks to access to easy to come by TMs: if it can't hit the opponents super effectively, then stay sure Magmar will still hit them with normal effectiveness.
Gym #3 - Whitney (Goldenrod City, Normal-type): If properly trained, Magby can fight Clefairy without any problems, barring of course bad luck with Metronome. Miltank, on the other hand, is a strong no: Magby's physical bulk is low and a STAB Stomp from Miltank will hurt. A lot.
Rival (Burned Tower): Haunter is doable, Zubat goes down to ThunderPunch (which is a must on Magby for coverage purposes), Magnemite and Bayleef fall to Fire Punch, Quilava might not be worth the risk and Croconaw is better if left to someone else. Watch out for Curse and confusing moves (and PoisonPowder, if you chose Totodile).
Gym #4 - Morty (Ecruteak City, Ghost-type): Magmar, once trained, can hurt the ghosts with STAB Fire Punch, just be careful of Shadow Ball coming from Gengar and shenanigans due to Curse, Mean Look and Hypnosis+Dream Eater combo. Magby can face the Gastly; keep an eye open against the Haunter and switch to another teammate if Gengar enters the field.
Eusine (Cianwood City, Crystal only): By now your Magby should have evolved into Magmar, if not, do it. A physical move like Dizzy Punch, Strength or Return will work fine on Drowzee and Electrode (watch out for Hypnosis and STAB Thunder respectively); Haunter can be easily dealt with the use of Fire Punch.
Gym #5 - Chuck (Cianwood City, Fighting-type): While Primeape can be easy to defeat, his STAB Karate Chop with a high crit chance against Magmar's physical Defense will make you consider resorting to another Pokémon. Poliwrath should be avoided, even if you have ThunderPunch, because Surf is super effective and Dynamic Punch hurts a lot and inflicts confusion.
Gym #6 - Jasmine (Olivine City, Steel-type): Fire Punch spam works like a charm. Be aware of Steelix's Rock Throw, anyway. If you want to be sure to take it down, send a Water-type in order to bait Steelix to set up Sunny Day.
Rocket Executive battle #1 (Team Rocket HQ): You can make it easily, but watch out for the dangerous Selfdestruct and STAB Hyper Fang.
Rocket Executive battle #2 (Team Rocket HQ): Fire Punch will take care of everything. If you need to switch while facing Murkrow, remember it knows Pursuit.
Gym #7 - Pryce (Mahogany Town, Ice-type): Another Fire Punch spam, another win; ThunderPunch is a better alternative against Seel and Dewgong though.
Rocket Executive battle #3 (Goldenrod Radio Tower): Too many exploding foes, too little physical bulk. Leave this battle to someone bulkier.
Rival (Goldenrod Underground): Sneasel, Magnemite, Meganium are easy peasy; Golbat and Gengar will fall to Fire/ThunderPunch; a physical, non fighting type move is the best choice for Kadabra. Be careful against Typhlosion and avoid engaging Feraligatr: it will retaliate with Water Gun and Slash.
Rocket Executive battle #4 (Goldenrod Radio Tower): Just like the previous battle in Mahogany, spam Thunder and Fire Punch to victory.
Rocket Executive battle #5 (Goldenrod Radio Tower): Houndour and Houndoom can be dealt with ThunderPunch, while the Koffing can be Fire Punched without risks.
Suicune (Bell Tower, Crystal only): Sending a Fire-type against a Water-type legendary that knows BubbleBeam AND Rain Dance? Are you crazy?
Gym #8 - Clair (Blackthorn City, Dragon-type): Magmar can't really do anything against dragon types, especially against Kingdra. Sure, it can spam a physical move against the Dragonair (just watch out for Slam and the lead's Surf), but it would be better to resort to someone else.
Rival (Victory Road): You should know the drill: Fire (or, better yet, Flamethrower if Magmar has learned it) and ThunderPunch everything, physical move on Kadabra, keep an eye on Typhlosion and avoid Feraligatr completely.
Elite Four Will (Indigo Plateau, Psychic-type): Jynx and Exeggutor fall to Flamethrower, the Xatu can be one- or two-shot by ThunderPunch; Slowbro is bulkier and can set up with Curse and/or Amnesia, so be warned.
Elite Four Koga (Indigo Plateau, Poison-type): Flamethrower on Ariados, Forretress (once it stops/fails with Protect) and Venomoth. ThunderPunch on Crobat. Muk can be hit with Flamethrower as well, but it will try to set up Minimize and will retaliate with STAB Sludgebomb. Also, watch out for poison, confusion and sleep.
Elite Four Bruno (Indigo Plateau, Fighting-type): Avoid Machamp because of high crit rate STAB Cross Chop and Rock Slide, Onix has low Special Defense but knows Earthquake and Rock Slide. Hitmonlee's Special Defense is pretty good and Magmar risks being hit by Hi Jump Kick, Hitmonchan and Hitmontop can be dealt with easily (watch out for Hitmontop's Dig).
Elite Four Karen (Indigo Plateau, Dark-type): Umbreon will be annoying (X Accuracy can help with that) but it will fall over time, Flamethrower goes on Gengar (it has Destiny Bond, so beware if it outspeeds) and Vileplume, ThunderPunch on Murkrow and Houndoom (whose Crunch can be nasty, especially if it rolls a Special Defense drop).
Champion Lance (Indigo Plateau, Flying-type): Gyarados is twice as weak to ThunderPunch (and its Surf shouldn't be much of a threat because of its subpar Special Attack, but switch out if it sets up Rain Dance; also, Flail can wreck you if it's used at low health), Charizard can be dealt with it as well, but Aerodactyl will outspeed and use STAB Rock Slide, so better avoid this matchup. The Dragonites are bulky, the best move would be to switch out Magmar for a teammate better suited for dragon slaying.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): You can't really do much, maybe use ThunderPunch on Kabutops and Omastar, that's it.
Rival (Mt. Moon, optional): Same strategy used in Victory Road, remember Kadabra is Alakazam now, so watch out for its Psychics.
Gym #10 - Misty (Cerulean City, Water-type): Higher level and ThunderPunch (Quagsire being the only one immune) or not, let Magmar spectate and nothing else, better safe than sorry. You could resort to Sunny Day if you are that desperate.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): The level difference (assuming Magmar followed you to the Hall of Fame) should be enough to one shot or almost everything, be careful of the exploding Electrode and you'll be fine.
Gym #12 - Erika (Celadon City, Grass-type): Look at Magmar's typing and moves. Now look at Celadon's gym type. GG Erika.
Gym #13 - Janine (Fuchsia City, Poison-type): Flamethrower on Venomoth and Ariados, the rest can be dealt with by the Psychic TM.
Gym #14 - Sabrina (Saffron City, Psychic-type): Flamethrower and a strong physical move (Return being the best candidate) are your best choices. Alakazam can be a problem, especially if it sets up Reflect, so stay always at high health.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Magcargo has Rock Slide and Magmar's Defense isn't that high; the rest will go down to ThunderPunch or Psychic.
Gym #16 - Blue (Viridian City): Rhydon is better left to another teammate, Alakazam is like Sabrina's (only higher leveled), Gyarados will go down to ThunderPunch, but it knows Hyper Beam, so it's a risky matchup. Arcanine is bulky and Extreme Speed can hit hard, Pidgeot is pretty simple to KO thanks to ThunderPunch and Exeggutor falls to Flamethrower.
Rival (Indigo Plateau, optional): Depending on Magmar's level, Crobat and Alakazam may or may not outspeed. The rest is pretty much the same. Remember to avoid Feraligatr once again.
Red (Mt. Silver): Pikachu and Venusaur can be taken out without problems, Charizard falls to ThunderPunch but beware of Slash, and Return is the perfect candidate to KO Espeon. Blastoise is a no for obvious reasons, while Snorlax has the bulk and moves to withstand your attacks and retaliate with Body Slam on your not-so-good physical bulk.
Wild Magmar already start with Ember, good only to inflict low damage for catching purposes, Leer, which is more of a support option since almost every level up move is special, Smog, the only physical move (which has low damage output) by level up and Fire Punch, the best this Pokémon will have for a while. Magby learns those four moves at levels 1, 7, 13 and 19 respectively (although Fire Punch is already available via TM). At level 25 comes SmokeScreen, but the RNG isn't something to rely on every time (or any time at all); Sunny Day is next at level 33, useful to boost Fire-type moves and nerf Water attacks; Flamethrower is learnt at level 41, a move that speaks for itself; Confuse Ray at level 49 can be an option if you like to confuse your opponents; Fire Blast, at level 57, is the last move, strong, but with shaky accuracy.
In Crystal, the Magby hatched from the Odd Egg will know Dizzy Punch, a solid physical attack whose power will be only rivaled by Goldenrod's TMs at the time. After beating the League, Flamethrower is also available via Move Tutor, in case you accidentally deleted it from its moveset.
TM wise, Magmar has many options. ThunderPunch is available even before catching it, offering coverage against Water- and Flying-types. Iron Tail, Headbutt (if you like flinchhax), Return and Dynamic Punch (despite the low accuracy and PP) can make use of its pretty good attack stat, while Psychic is another strong special move that offers coverage against Fighting- and Poison-types and can lower the enemy's Special Defense; provided it has good typing and base damage, even Hidden Power is an option. For supporting purposes, Curse offers the physical boost you might need; Toxic, Protect, Endure and Detect are useful for stalling, Attract works well if taught to a female (since most of the trained foes are male); Swagger may be considered, just remember it raises the foe's Attack by two stages and Magmar's Defense isn't good at all.
Grass types: Magmar greatly enjoys Grass types (or teammates who can access Grass type moves), as they're the weakness shared by the types Magmar is weak to: Rock, Ground and Water. Provided the opponents lack Fire type moves, Magmar can, in exchange, set up Sunny Day for an immediate and consecutive use of Solarbeam.
Water types: Water types, other than countering Rock and Ground types, deal faster than Magmar with other Fire types, while the Fire duck can in return ThunderPunch opponent Water types (naturally, that's assuming Magmar itself isn't in danger of being KO'd). Plus, access to Ice type moves allows them to be switched in for Dragonslaying duty.
Physically bulky Pokemon: Due to Magmar's low physical bulk, physical walls, or Pokemon with a remarkable physical bulk, are helpful switch ins against an opponent who's packing an high attack stat and/or powerful physical moves.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Magby Line Ratings
At what point in the game should I be evolved? In Crystal, training makes sure your Magby will evolve by the time you have reached Cianwood; if that's not the case, it is advisable to make it evolve before facing Eusine and Chuck. Gold and Silver players can catch Magmar already evolved.
How good is the Magby line in a Nuzlocke? It's pretty good, especially since it can be obtained pretty soon, it's a fully evolved Pokémon (if it's been caught in Gold or Silver)/doesn't require an elemental stone to evolve, unlike Growlithe/Vulpix/Eevee (if it's been hatched in Crystal), and has easy access to coverage moves. Just stay clear of the usual weaknesses and any physical attacker and you'll never grow bored of Magmar.