Best Pokemon (in my opinion) to get would be:
Chimchar (only fire-type in-game before post-game apart from Ponyta)
Actually, you can get houndour before you even get to sunyshore. Houndoom is very useful against the elite four when you EV train it properly.
Knost, if you haven't started yet, I'd suggest picking torterra and trying to catch houndour for your fire type. Houndoom's flamethrower (learned naturally at level 48) can kill Aaron's entire team in one or
maybe but probably not two hits, not counting drapion. Drapion isn't actually a bug type, but poison/dark, so torterra can definitely OHKO it with earthquake (learned at evolution)
A water type is extremely helpful for bertha/flint as it will deal super-effective STAB damage against all of flint's team and 3/5 of Bertha's pokemon. I can't remember any good water types being available in diamond before getting the national dex, so I'd suggest to just go with a gyarados, which will be very easy for you to find.
I beat platinum with just these three; as long as you let them learn the right moves it should be easy for you. My team at the time:
Torterra-
Earthquake
Crunch
Synthesis
Woodhammer (requires a heart scale; gotten from the underground or the man in the house in Solaceon town. If you don't want to use one, razor leaf will work, but isn't nearly as powerful)
Houndoom- I suggest letting Houndoom hold the choice specs for aaron/cynthia to boost its special attack. You get them from a man in the house in celestic town (the one where you buy medicine) between 4:00 and 10:00 (AM). If it's any other time of day you'll get a different item. They're amazing for this situation because you'll only be using one move anyway. I also suggest you EV train houndoom in speed, as there are some fast enemies in the elite four and you need to outspeed them to get in your one shot STAB flamethower, boosted by choice specs, because if they go first they might one shot you.
Flamethrower
Howl
Roar
Dark pulse (found in victory road, you can use crunch if you dont want to use the tm but it wont be as useful as houndooms special attack is dominant over his physical attack)
Gyarados-
Bite
Ice fang
Dragon dance
Waterfall (gotten as you progress through the game, can let it learn aqua tail temporarily but waterfall is better IMO)
These pokemon should be able to take you through the game alone, the other three can be whatever you want. I know the movesets aren't very good, but these are what mine were at the time and I saved most of my tms.
Like I said before, you should be able to flamethrower most of Aarons team, leaving only drapion to be killed with one earthquake. For bertha, start with torterra, using woodhammer/razor leaf on whiscash, whichever you taught it, because it's 4x effective on the Water/Ground combo. After that, sweep her entire team with Gyarados. Ice fang is 4x effective against Gliscor. Waterfall will be 4x effective against golem and rhyperior. Hippowdon isn't much of a problem.
For flint, keep using gyarados (make sure you start the battle at full health, as you'll be using it a lot) Waterfall is super-effective against all 5 of Flint's pokemon, so this shouldn't be hard.
For lucian, you'll have bite on gyarados and crunch on torterra but you defeinitely want to be using dark pulse/crunch with houndoom, as houndoom is much faster than torterra and gets STAB on dark moves. Not to mention psychic moves have no effect on dark types. Note: Don't equip the choic specs for this fight, as you'll need to alternate between dark pulse on mr mime/espeon/alakazam and flamethrower on bronzong (which has levitate, so dont try to earthquake it) and the only enemy houndoom has to fear on this team is Gallade, as it is part fighting, meaning not only are dark moves normal effective (rather than super-effective) but it has the potential to OHKO houndoom. Switching to torterra and earthquaking it would be your best bet.
Cynthia shouldn't be too hard. Make sure your pokemon all have full health if you bought potions, spiritomb has no weaknesses but it'll deal the least damage to houndoom so you can use flamethrower there. If you followed my advice and gave houndoom the choice specs, it should be able to kill roserade and lucario in one flamethrower (supereffective) but you'll need to be fast enough. Gyarados can take down togekis with a couple ice fangs. Milotic will be your only problem. Torterra is strong enough to kill it with grass moves, but far too slow to outspeed it, and with a low special defense and 4x weakness to ice you would probably get killed by milotic's ice beam. If you have 3 other random pokemon, you can probably finish one more pokemon though.
So that's it... I beat my first Nuzlocke like that, just make sure when you're training dont take too many risks. If your health is low, heal. You never know when a miss/critical hit will ruin your whole run, but it can happen. Good luck!
