AKA the "Throw Mightyenas at it until it dies" run, or the "Timber you suck enough at Reborn in a normal playthrough, why make it even harder?" run.
This isn't any kind of how-to guide so much as it is a log of my own experiences in this challenge, which may help anyone who wishes to try it themselves. Arceus only knows why you would, though. (Also feel free to move this if it's in the incorrect section.)
The rules I've set for myself are:
-Only Dark-type Pokemon are allowed (naturally). All other Pokemon are illegal.
-Dual-typing is permitted. If a Pokemon is not Dark-type, or never gains that typing, it is illegal.
-If a Pokemon is not Dark-type, but gains that typing in a higher evolution, it can be used. That means Froakie/Frogadier, Skorupi, etc. are permitted even though they are technically not Dark-types. Mega Evolutions do not count, as we'll probably never be able to use them, so sorry Vinny. No Gyarados.
-Illegal Pokemon may be caught and trained for breeding purposes, but they may only battle wild Pokemon i.e. battles with no bearing on game progression. If an illegal Pokemon is in my party and I run into a trainer, that Pokemon may not participate in the battle for any reason, not even switch training. If illegal Pokemon are my only surviving mons, I'll consider the fight lost. If I accidentally enter a fight with an illegal Pokemon as my lead, I must restart regardless of when/where I last saved.
-HM slaves are allowed, but they must abide by the above rule.
Those rules are probably very lenient. I'm sure some hardcore players probably wouldn't permit the use of illegal Pokemon for breeding or HM slaves, or allow the use of Pokemon that don't initially have a Dark-typing, but I am far from hardcore. In fact, I am so un-hardcore that it's counterproductive put myself through something to make Reborn even harder. I'd like to say it's for the sake of doing something different to keep myself occupied, but I think the real reason is that I just plain hate myself.
So! Why Dark-types? Good question, actually--I guess they're just cool. Of course, even before I actually started, I was already planning on which (currently available) Pokemon I could use. Dark-types offer a decently wide range of dual-typing to cover plenty of situations, which is good news for me because just Biting everything to death will get boring after a while. Unfortunately, Biting will be all the rage for the first chunk of the game.
I'll probably only talk about battles I found particularly notable or difficult. I'll definitely discuss the gym leaders, but I'll only discuss rival and Meteor battles if I needed to go out my way and strategize to beat them.
Starting out
I was strong-armed into picking Froakie since it eventually gains the Dark-typing. Of course, if I didn't have the spine of a Tentacool, I'd have abandoned the poor thing the second I got a Stunky or Poochyena. Because really, those two Pokemon were my only options. Them and Purrloin, but Purrloin is... yeah. I figured Froakie's Protean ability would be key to winning many battles, but honestly, as of this typing, Froakie/Frogadier hasn't done me much good. My front liners so far have been Mightyenas.
Fighting Fern and storming Team Meteor's factory was a little tedious, but nothing terrible. Bite is a decent STAB move and its Flinching side effect helped me out at times. They didn't truly come in handy until they evolved into Mightyenas and gaining Intimidate/Moxie.
Julia
So far I've been relying mainly on Poison stalling strategies with Stunkies, but Julia's team can't be stalled for long without them becoming dangerous. Her team spams Charge Beam, which raises Sp.Atk with each use. That means Intimidate does little good, since they mainly use special moves. I managed to sweep her whole team with just a decently-leveled Mightyena. It didn't have Moxie, but I used Howl enough times where it could faint her mons in one or two hits. Her Electrode was dangerous, but manageable.
Really, it was a fight I just had to brute force. I had to be economical with my moves, too. Sonic Boom could knock my mons into the yellows or reds, and each Charge Beam used meant greater and greater danger. Many of her team had Static to boot, meaning each time I used Bite was a chance I could get paralyzed. I couldn't find any safe openings for my Stunky to get in any Poison Gasses or Screeches. I had to hit hard and fast and pray for a little luck.
I'd discuss this fight more in depth, except I don't remember everything that happened and honestly, it really was just me throwing a Mightyena at her and hoping for the best. But I will say that it was the first time I fought Julia that took multiple attempts, since I was testing the waters to see what could handle what. And it turned out Mightyena could handle everything.
Fern (before Florinia)
Of all the Fern fights, this one always seems to give me the most problems, because his team is very meaty and cheap for this stage in the game. I had trouble with him when I had Pokemon with an advantage, let alone all Dark-types.
This is the time where I obtained multiple Migthyenas, as a single one couldn't cut it. Some of them had Intimidate, some had Moxie.
I opened with Intimidate Mightyena to cut Lombre's attack a bit. Lombre's Tri-Attack hurts and can cause status changes, and many of my restarts came from it spamming that move twice in a row. Hope that it uses Fury Swipes or Absorb instead, as they don't particularly hurt--the former tends to miss a lot or not connect many hits, and the latter doesn't do much damage. Fake Out is a minor annoyance at best. Use this chance to stock up a Howl or two, and a Bite should be enough to one- or two-shot it. Try to make Fern waste his Super Potions here. You do not want him to use them on his Servine or Roselia.
Servine was very meaty and annoying, and Intimidate was pointless since all its attacks were special. I had to pray it didn't have Contrary and switched to Stunky for one or two Screeches. Stunky didn't survive long enough for me to do more. Two Leaf Tornadoes was enough to faint it, and that's without Servine spamming Growth. Sometimes I had to make do with one Screech and one Poison Gas, although I had to hope both of them connected, as they don't have 100 percent accuracy. I waited for Stunky to faint since I couldn't risk a turn switching it out, and hammered with Servine's defense lowered (and poisoned, ideally), I hammered it with Mightyenas. If I found an opening, I risked a Howl to raise its attack further.
A Howl served me well since Roselia was fragile in comparison. Since it was part Poison-type, I couldn't rely on a Poison stall strategy, so brute force saved the day.
Florinia
Hoo boy, Flobot was a hell of a brick wall. Surprisingly, it wasn't her infamous Cradily that gave me problems, but her part Fighting-type Breloom. This fight took me multiple attempts to see which Pokemon could handle what, for how long. I got another Stunky as backup, because no one Pokemon could last forever in this fight. I relied mostly on Poison stalling, Screeches, and Intimidates, and enough luck to get those attacks in before fainting.
I opened with Intimidate Mightyena to lower Cacnea's attack. I immediately switched to Stunky to get in a Screech (one was enough). Cacnea usually uses Sandstorm, a minor annoyance at best, though Pin Missle can hurt if it connects enough times. Luckily, Stunky's part Poison-type means Bug-type attacks are neutral against it, and neither it nor Grass Knot will do significant damage with Cacnea's attack lowered. When Stunky got in a Screech, I switched back to an Intimidate Mightyena and one-shot it with Bite.
When Flobot switched to Maractus, I switched to a different Intimidate Mightyena to lower its attack. It's quite important because Maractus has Pin Missle as well, which is super effective against Mightyenas, though not against Stunky. After the Intimidate, I switched to Stunky for a Screech. Since it most likely took damage from Cacnea previously, it'll most likely only last long enough to get in a single Screech, which should be enough. You can either wait until Stunky faints, or you could switch it out without wasting a turn (I managed to switch it out with 1 HP left), and switch back to Intimidate Mightyena, lowering its attack further. Bite wasn't enough to OHKO it, and I didn't want to risk setting up a Howl, so I used this mon to waste Florinia's Super Potions. After that, it was only a matter of one final Bite.
Then it was Breloom. God damn it, Breloom, with its priority super effective Mach Punch. Even Stunky, who isn't weak to Fighting-type, can't survive too many of those. I got in a Screech (though it'll probably faint by that point, but that's what your backup Stunky is for!) and then I switched to another Intimidate Mightyena (I had, like, three btw), and the first thing I did was Roar it out of the battle. I needed to whittle down its strength and defense before I was ready to fight it.
After Roar, Florinia will either send out Cradily, Phantump, or Grotle. If it's Phantump, congratulations on encountering one of the blessed few Pokemon you're super effective towards. Bite that thing and move on. Grotle isn't bad, either. It will probably summon a Sandstorm to buffet itself, the poor dear, and two or three Bites will be enough to down it. Be wary of Razor Leaf. If you didn't get Florinia to waste her Super Potions on Maractus, Grotle is another safe option.
If it was Cradily, I immediately switched to my backup Stunky for Poison gas. I didn't bother with Intimidate or Screech since Cradily likes to spam Curse. Its only offensive move, Smack Down, will hurt, especially with multiple Curses in place. I found it was a matter of me outlasting it. I healed after each Smack Down, and when I thought I had an opening, I used Bite. Be glad if it causes Flinching, or that Cradily doesn't use Recover. Eventually, it'll fall to Poison damage.
After beating Cradily, Phantump, or Grotle, Florinia might send Breloom back out again. If she tries, send out an Intimidate Mightyena and Roar it back out. Mach Punch hurts and there's no safe time to set up Howl. After about two Intimidates, it should be safe to send out Stunky for Screech. The Screeches are very necessary since Bite, which Breloom resists, is Mightyena's strongest attack. With enough Screeches in place, though, even Bite will take a huge chunk out of its HP.
Take your time, whittle down their stats, and pray for the RNG to be on your side, and take sweet sweet victory.
Taka fight #1
I beat this guy on one try.
Not easily, mind you. That Chatot brought some casualties, but another Poison stalling was enough. A Howl or two from my Mightyenas was enough for Lileep and Tangrowth.
Off to play moar!