DiamondDialga

Ahh, Purity Forest. Easily the hardest Dungeon in the game And the reward for this incredible, 99-floor, level-1 regression, no items/money/teammates/recruiting dungeon, is... *drum roll* the great Time-Travel Pokemon, Celebi. It takes an incredible amount of, in the words of that chicken from Chicken Run, "hard work, perseverance, and... hard work".
Now, my friend, sit back and prepare to learn...

The Ways of the Forest

First off, I would like to say, always have someone who can save you. I don't care HOW strong you think you are, you WILL be defeated. 99% guarantee. I recommend using dungeon and waiting overnight for someone to reescue you.
The Heroes

The first thing you must know about Purity Forest, is who has the greatest ability to get through. You must choose a Pokemon, and choose wisely.
The heroes of Purity Forest:

Charizard
At level one, Charizard knows Heat Wave, a Fire-type move that hits everything in the room. Which is helpful when you remember that Purity Forest is filled with Grass- and Bug-type Pokemon. Problem is, he doesn't level very fast.
My Rating: 6/10

Electrode
At level one, Electrode knows Sonicboom, a 55-set-damage ranged move. In level-one regression dungeons, these will save your tail. Guaranteed. He also levels at an okay rate.
My Rating: 8/10

Alakazam
This guy is who I actually beat the dungeon with. He has the best of all level 1 stats, and has a nice moveset to boot. (Teleport can help in most sticky situations. Heck, if I hadn't used it, I probably never woulda survived floor 98 and gotten to the stairs.)
My Rating: 8/10

Mew
He levels insanely fast, and can learn ANY TM you find. That's right, ANY TM. Guaranteed. His problem is that his starting moveset consists of Pound. Nothing else.
My Rating: 7/10

Mewtwo
He levels at about the same rate as Mew, but he has a better starting moveset. However, for this, he sacrifices the ability to learn every TM.
My Rating: 7/10

Kecleon
I haven't actually USED Kecleon (because a certain green lizard I've killed over a thousand of REFUSES TO JOIN ME >.>), but I've heard some good feedback from people who have. He levels at a great pace, learns a lot of TMs, and has a great starting moveset.
My Rating: ???/10

The Artifacts

As you cannot bring any items into the dungeon, getting through this forest will require some luck. Here are some items you should gladly die defending.

Pass Scarf
I cannot stress how helpful this baby is, if I hadn't found it, I never would have made it through. It whips most attacks right back at an enemy (but only if they're melee attacks, like normal attacks, Scratch, etc.). This can cause most enemies to eliminate themselves very quickly. With such a great item, there's got to be a downside, right? Right. This one's downside? It decreases your hunger as you use it. And when you're out of Belly, it won't work. (Hunger's more dangerous than you may think; I died on floor 79 from hunger.)
My Rating: 10/10

Warp Scarf
If you're not using Alakazam, or he's out of PP for Teleport, and you're in a monster house, that's when this baby shines. Use it, start to run away, and be teleported. And when you get tired of the downsides (see below), toss it at an oncoming super-strong enemy, and run. He'll get teleported while hunting you down!
There are two downsides to this guy, though. One, when you use it to escape a monster house, they all split up in a crazy attempt to find you. When they find you, well, let's just say you'll be saying "cwap" while sitting on the ground waiting for your knight in shining armor. Two, if you forget to take it off, you'll continue to be randomly teleported every 5 or so steps. Which is annoying if you just found the stairs >_<
My Rating: 8/10

Def. Scarf
It increases your defense. No downsides. 'Nuff said.
My Rating: 7/10

Food
If you can't figure out why this is important, you have the IQ of a plant pot >.>
My Rating: 10/10

The Villains

No dungeon is complete without some worthy adversaries. This is a list of the best of the guys who want to smite you.

Togepi
Awww... so KYOOT! Until he starts smiting your attacks with Growl. MUST KILL EGG >.> He also knows Metronome, and that can turn out as any move. (I'm pretty sure he can also pull out OHKO moves with it. ) When he gets lucky with Metronome, you're doomed. Also, WHAT THE HECK IS UP WITH HIS DEFENCE? >_<

Jigglypuff
Two words: Cute Charm. No, I DON'T want to be infatuated while you pound me into the dust, thank you very much.

Electrode
Screech is absolutely deadly. Murdered defence, anyone? >_< Let alone the fact that he has Sonicboom, which can get through Pass Scarves, and kill you in about 2 hits. I've never seen them use it, but they likely know Explosion, too. AVOID THEM.

Natu
Tough to kill, and give a pathetic amount of experience. No thanks.

Parasect
THE most annoying Pokemon in the game. If you disagree with me, you haven't been through Purity Forest. Spore spammers, deadly moves, etc., etc. AVOID AT ALL COSTS.

Breloom
Dear Aruseus, these guys are annoying. They have the Effect Spore ability, giving you a random status problem when you attack them directly. The majority of their moves also deal status problems. Avoid them, unless you like paralysis/sleeping/poison/burns/whatever.

Venomoth
"Venomoth uses Silver Wind!
Alakazam took 237 damage!
Alakazam was defeated!"
Venomoth is tough, Nothing said.

Dugtrio
Magnitude will SMITE you in 1 or 2 hits. Easily. Avoid at all costs.

Hitmonchan
He knows Agility, which makes running away a bit tougher... Oh, who am I kidding, running from a double-speed Hitmonchan is like trying to row a boat in a cornfield. When he uses it in monster houses, you can kiss your sweet tail goodbye. And as if that's not enough, he knows Mach Punch, making running away even more impossible.

Ralts
Annoying for the same reason as Togepi. This piece of psychic broccoli can murder your attack. Which is bad.

Kecleon
You should have the brains to avoid stealing from this guy's shop, hopefully. If not, you won't last long. Do NOT, I repeat, DO NOT, and just for emphasis, DO NOT steal from Kecleon shops. If you do, they can and will kill you in a single hit.

Anything to do with monster houses
'Nuff said.

Never Use These Items

Some artifacts help you. And others don't. Which don't? These ones, mostly.

Weather Orb
Random weather? Well, let's think. Hail and Sandstorms do damage to you every few steps, Sunny weather hits Chlorophyll in some of the enemies and speeds them up, Blizzard does nothing, Rain may increase some Pokemon's speed, Fog demolishes your attack power... Need I go on?

Double-Edge Orb
Halves your health to leave all enemies in the room with 1 HP, if it hits. Sounds good, right? It sounds like it's an all-or-nothing deal. You hit them, everyone's left with 1 HP, you have half HP, kill them off with Heat Wave, right? Ahahaha-no.
It actually hits each Pokemon seperately. For example, say you're in a room with 5 enemies. You have 200 HP when you pull out a Double-Edge Orb. It hits the first enemy, they have 1 HP, you have 100. Hits the second enemy, you have 50, they're barely alive. Misses the third enemy. Hits the 4th enemy, you have 25 HP. 5th enemy gets hit, and you're left with 13 HP. Now the enemies can probably kill you before you can kill them. Soo, yeah. Avoid that.

Escape Orb
I've heard horror stories about people who were rocking Purity Forest by simply throwing Silver Spikes like a madman, then when they run out, accidentally use an Escape Orb on floor 98. But that's besides the point. This is my point.
When you use an Escape Orb, you're essentially giving up. You come so far into the dungeon, and then throw it all away because of that little bit of worry that you won't make it. Courage, my friend. Perseverance.
Quitters never win, pal, and winners never quit. What do you want to be, soldier, a winner or a quitter? Do you want to be known as the guy who never gave up and got the 251st Pokemon, or do you want to be known as the guy who gave up out of fear?
NEVER use an Escape Orb. Don't even keep it in your inventory. Don't give yourself an opportunity to quit.

Traps:

Breif bit on traps, HATE THEM ALL accept for Wondertile, who saves you from Togipi, Broccoli, etc

Well, this sums up my guide to Purity Forest. Good luck, and remember: Never give up.

Piczo