All right, here it is. A starter to a guide on how to take out the true final boss of the game, the Living Fortress Maple. You want something on the rest of Maple Tree, you know where to look. And you probably should, or you'll wipe in Form 2.
General tips:
Okay, first off - the thing everyone knows about, and most people still screw up with - Maple's Poison. Do NOT expect your healer to handle it. Maple tends to cast it a lot, and it leaves persistent AoEs. Your healer will waste valuable time and MP trying to keep Antidotes up if you let them handle it. Poison Immunity potions are much better, but not idea. They are expensive, only last an hour, and prevent the use of Paralyze Immunity potions. Better to get the Poison Immunity Skill, and let the healers pop Paralyze Immunity potions and clear that debuff as needed.
The trick for getting the Poison Immunity Skill is that it actually has to cause damage. So get yourself of pile of healing potions and a healer or two, find some Triffids or go to the Bog of Eternal Stench or whatever is appropriate, and get yourself poisoned. And keep popping potions until you reach at least Medium Poison Resistance. Once there, heal and cure yourself, restock with at least 99 potions, and go to the Poison Dragon's Labyrinth back on Level 1. Instead of interrupting or stunning the Poison Breath attacks, take the hit and start popping potions immediately. This is the fastest way to go from Medium Resistance to Immunity.
The other issue with the Poison is that it does a number on your gear's durability. Unless you have unbreakable, get a couple copies of your non-accessory gear and weapons, or you will find yourself naked mid-fight.
Second, pick where and when you fight her. You want her to come to you, not the other way around. Ideally, you want something to discourage her from just flying away, enough open space to place all of your parties, some cover so she doesn't realize immediately how many parties you brought, and for it not to be on a hill or have cliffs nearby. If things are going bad, she WILL surround herself in a poison bubble and attempt to roll away much faster than she would normally be able to move. Finally, try to fight her in an event that will 1. get you rewards for winning, and 2. slow down respawns so she and her pals are not coming back immediately.
Third, clear out some of the Maple Tree adds before the fight, or Form 2 & 3 will have too much DPS to survive. Sally the Blue Devil is almost as dangerous as Payne, and you WILL wipe if she joins up with Maple. Furthermore, fighting her requires a completely different set of builds then Maple. Get a second raid to deal with her, see the guides. Lightspeed Kasumi is a single-target PvP specialist. See the guides. Undying King Kuromu, see the guides, remember he had a 50% of showing up anyway. These three are the major ones. You can target Puzzle King Kanade, but causing him to burn through his instant-cast spells a day or so before you fight Maple should be enough. Iz is a trapper and bomb expert, but not too dangerous, and do NOT take out Mei and Yui - they're needed to cheese Form 2.
EDIT, tip from Gandalfr: Fourth, you probably want some swordsmen with Shear and some mages with fire spells. Every now and again in Form 1 she will cast Wooly, followed by Hydra. You want to either cut the wool or burn it before she casts Hydra. If she does cast, the person Shearing her will probably break their weapon, and burning it tends to make Poison Mist AoEs.
Actual Fight: Maple has a variety of Forms she goes through during a fight. The only hope of beating her is to adjust to each Form and burn through it until she has no more Forms left.
You want a variety of parties, including - trap party for Form 1. High damage/damage pierce AoE party for Forms 2 & 3. High damage/damage party single target party for Form 3. Dodge tanks and VIT tanks with high Provokes to perform kiting and avoiding Counter cleaves, with healers and buffers in equal amounts. And a final defense party that can shield the other parties for Form 4. Add additional DPS groups as long as you can hide them and shield them with the final defense party.
Don't bother with status effects - she has a ton of immunities and gets more randomly. Traps can be low level as long as they do not target her VIT stat. DPS needs as much damage pierce and actual damage as possible, with as much stuff to repeat it as possible. Maple has an obscene defense and can heal herself very easily, but has low total HP in most cases.
The actual Forms we know about are as follows:
Form 1 - Default form. The normal Maple, and actually a bit of a puzzle boss. As always, you want to avoid close range combat until she burns through all 10 devours, or you will lose someone and grant her extra MP charges. The trick here is you want to cause her to go to Form 2 without triggering either of the other Forms. This Form has 0 Agility and 0 Strength, so we actually suggest ranged binds and traps. Once she is successfully pinned, she will summon her adds and go to Form 2. Do not bother with doing damage at this point to avoid Form 3, and try to keep your numbers down or she might go Form 4.
Attacks are Predator demons from her shadow, her pet Syrup (see Syrup), various Hydra poison attacks, AoE paralyze, Shield Bash, and Devour if she has the black shield out.
Form 2 - Angel Maple, also known as the Add phase. Maple turns into an angel, takes a hit to her HP (possibly the biggest single decrease you'll see all fight), and starts up her AoE Defense. She also summons Adds to help her. The Predator demons are still here and annoying, and Syrup is a possible target, but you should really hope for Mei and Yui (and maybe Kanade and Iz) to show up to break her free.
The reason why is that the angel skill, "Loving Sacrifice", causes Maple to use the skill Cover on all of her allies within the field, sharing her defense with them. This means that any attacks that do damage to her allies does double damage to Maple instead. Mei and Yui have only gear VIT, so having just enough AoE damage to do 1 point to them and Maple actually means doing 5x damage to Maple. With the initial HP hit, this is a great time to drive Maple's HP as low as possible before Form 3.
No changes to her attacks, but you do need to watch out for her adds. You want her HP to go down fast enough that she switches to Form 3. Too slow, or too many people in sight, she goes to Form 4 instead.
Form 3 - Demon Kaiju Maple. Nastiest phase known. Tends to go into it when suffering from major HP loss. The form is giant, cleaves in front and behind, has a breath attack, has her stats plus mid-tier player's Agility and Strength, and a comparatively huge HP bar. There are only two bits of good news. First, it seems to have a long recast, possibly a once-a-day skill, so you only have to deal with it once. Second, none of her equipment seems to work in this form, so no combination with Form 4. You want your tanks to point her perpendicular to your DPS, and to kite her around to keep her occupied. The AoE Defense should drop (unconfirmed), so you should clear out the Adds ASAP once this Form starts. If you did your prep right, your AoE party from Form 2 should be able to handle this. Meanwhile, the rest of your DPS should get to Maple's flanks and start hitting her. This is a good time to attempt the Syrup loophole as well (see below).
There are two ways to force Form 3 to end successfully - burn through all her HP, or show up with enough forces at this point for her to decide Form 4 is better. The bad news is once the form is over, she is back in either Form 1 or Form 2, and can usually heal back to full before she's a valid target again.
EDIT: Note from ThorinSonOfOrin - Kaiju form can and will eat you if you ever get in range of its mouth. If you are lucky she doesn't chew. Stomach gives poison and typeless DoTs, and tends to wipe equipment in the first few seconds. Fighting your way out is futile. People eaten restore Kaiju's health and MP. I suggest kite-tanks for Form 3 since normal tanks are just lunch. I apparently taste like pecans. EDIT2: No Devour skill notification or gem, so this might just be an ability of the form.
Form 4 - Gundam Maple. For the fans, she's Dendrobium. The good news is this Form tends to only last a limited time. The bad news is she's launching multiple artillery strikes, bombing runs, and the like during the entire run. My suggestion is to have everyone bunch up, pop protections and cooldowns, and have the healers go ham trying to heal through it. Spreading out just increases the risk of someone not getting healed and being nailed by a beam cannon. Observations from the 4th Event seem to indicate she does have ammo limits, so this should be a phase you can wait out.
Oddly enough, Maple will rarely summon parts of Form 4 during Forms 1 & 2. These can drastically increase her DPS for a single strike, but tends not to be more than that. Current theories are that Form 4 is actually a special equipment set, with their own attacks and cooldowns per piece. Anyone has proof, please notify me.
Form 5 - UNCONFIRMED Turtle Maple. Rumors abound about Sally the Blue Devil doing illusion magic while having fox tails, and fusing with your pet is just something a game like this would do. If she can, and it just adds Syrup's stats and abilities, then you get Form 1 with some actual combat stats, even more defense, and HP and MP bars that are normal. Skillwise, she would be able to turn giant, fly, control nature to bind or impale you in an AoE, and fire the Kamehameha from her mouth. EDIT: Splitsword Shin mentioned that people would be about mouth-sized in the giant form, so she might eat you in this form as well.
Form 6 - UNCONFIRMED Unconfirmed Maple. Undead King Kuromu was noted to mention on the Great Shielder message boards that "We left Maple alone for a day and she came back with a crazy-ass skill. Again. And her humanity slips further away. Again. When will we learn?!?". While not proof Maple has an additional form, nor providing any actual details, I think it best to remind people that Maple always has one last trick no one knows about.
Anyway, after burning out the other Forms, you should get Maple back to Form 1 or 2, in which case you just have to finish off her HP. At least twice, since as a Great Shielder, she's bound to have somewhere between 1-3 skills that prevent dying the first time you should in a battle. The last thing you should worry about is Counter - which is why you need a sacrificial tank with a high-level Provoke skill away from anyone else just in case. If you manage to take down Form 3 the long way, Maple has probably absorbed enough damage to one shot your entire raid if she can catch them together.
Syrup Loophole: Submitted by DefinitelyNotDread. Observation that Syrup has lower defense than Maple, does not seem to get the defense boost from Form 2's AoE Defense, and reflects at least some damage back to owner (Maple). This might provide an alternative way to damage Maple. On the other hand, killing her pet is bound to piss her off.
