Etrian Odyssey 3 : Completed!

Last route cleared and hidden boss defeated! 

PS: Currently going back to my old old Etrian Odyssey 2 (English) and trying to complete the game. Like EQ3 and worse, I have not defeated the hidden boss. Heck, I haven't even defeated the Dragons. So I plan to start where I left off, but I am using a new team and all. Also need to collect the items and fill up the monster Dex. However, unlike EQ3, there is no Guild Card to prove these achievements. (Well, there is an item)

I thought about buying a new JP one, but I couldn't find a cheap one that shipped fast, unlike EQ3... 

I plan to get Monster Hunter 4.

Guild card upon defeat of hidden boss:

  

My party for defeating the hidden boss. For the last route, I simply solo-ed one of the below Zodiac/Monks. However, I did use a Phalanx as well against the story boss, since despite the level, a Zodiac is a Zodiac and cannot take many hits.

Details below, after pics:

Abyssal God, known in the JP version as Kuroki Kaien no Magatsugami (Evil God of the Dark Ocean Depths), is the hidden boss of Etrian Odyssey 3. Also likely based off of Cthulhu.
In a way, you can say this is actually the True boss of the game, due to its role in the story.
Description and then my party strategy afterwards:

Unlike the other bosses, Abyssal God pretty much has an invisible time limit. 40 turns. Once you reach this limit, all of his attacks become even stronger than ever. And his attacks are strong enough as it is. On top of it, he leaves his attack pattern and uses skills randomly.
On top of this, once you take out 1/4 of his HP, he also goes into random mode. Unlike the 40 turn version, this random mode does not see a stat boost, but it becomes impossible to predict his moves and thus far harder to block them.

So you pretty much have to take a strategy that kills him in the 1-2 turns you attack (2 turns if you can attack him first after your first attack).

Like Etrian Odyssey 4, this boss has two forms.
His first form weakens as you defeat the tentacle FOEs across the last floor. If you defeat them all, you can easily take out his first form in one turn. Without removing any tentacles, he practically negates all elements except Void. Unlike EQ4 though, it is easy to get by this form without damage as it only uses the three elements in a pattern. the 3 Guards will do the job here.
If you do get hit, you'll pretty much game over though. That's how strong the attacks are.

His second form switches between Defense and Offense modes. While in Defense, he has great resistance to every element while on Offense, he is weak to the 3 TEC elements. 
When he changes modes, he removes all debuffs on him. So in general, you buff your characters while he is in Defense and unleash your full force when he switches to Offense. You will have to cast your debuffs after he enters Offense, so you will need to survive his first attacks in that mode.

In Defense, he only uses one damage move the entire pattern. This skill can still do 200-300 damage to a Lv99 -> Lv99 Zodiac, but it is not enough to kill. The rest of the time, he uses Counters and healing moves, so you can leave him alone.
When he first switches to Offense, he uses a move that hits multiple times AND removes buffs upon contact. Even if you just use Taunt, it will attract the first hit, but that one hit will remove Taunt and the rest of the attack will spread out upon your party. This might be the best turn to use Aegis Limit skill to defend the party.
His next two attacks are one that damages the entire party and Stuns, and one attack that does massive damage to the person with the lowest HP.


So anyways, that is my basic analysis.
My party's attackers are two Zodiac/Monks (doped to max Strength). Thanks to the Zodiac class skill, their Breaking Fist can do even more damage than Monk/Zodiac.
Their job is to simply Etheric Charge -> Breaking Fist. One shot from each Zodiac is enough to take the boss out.

Their buffs come from the remaining party.
Shogun's Fore Honor not only boosts ATK, but allows the Zodiacs to go first.
Attack Order from Princess.
Boost from the ATK boosting item.

The three debuffs are:
Wildling's Primal Drums
Buccaneer's Eagle Eye
Gladiator's Wolf Howl
However, I don't really need Wolf Howl. It also takes two turns, since Wolf Howl is on the same character as Primal Drums.

Etheric Charge
Etheric Charge
Primal Drums
Eagle Eye
 ->
Breaking Fist
Breaking Fist
End