Toughest optional RPG bosses and a Bravely Default question

For those who've finished Bravely Default, I was wondering if you can do the Vampire Castle subquest after defeating Alternis at the Holy Pillar?. When I defeated him, I reset the game without saving, because after the battle when I ended up in the inn in Caldisla, I went to the overworld and noticed that the exclamation point by the Vampire Castle was missing. If I can do this side-quest after that fight with Alternis, is it better to do it after or before?
I've been gaining a ton of levels recently with more than half my allies job levels at least at level 10, but although my intent has been to get all job levels at 10 or higher, so far, fighting the dragons has still compared to levels in Silver Surfer.

Also, what are some of the toughest optional bosses you've fought in all the RPGs you've played?

Tales:

Abyssion. (if you don't solo him with a character that can dodge him or doesn't even have to, like Supress Presea)

Radiant Historia:

Mist Spider (if you don't follow a very, very specific pattern while fighting it)
Master Vainqueur (what is this I don't even... This is no optional boss, this is an optional case of Acute Total Party Kill-syndrome)

Golden Sun:

Dullahan (except if you use a Djinn to negotiate a freaking cease-fire and have elemental power raising equipment)

Kingdom Hearts: Sephiroth.

Megaman Battle Network: Bass

Tales of Vesperia: Kratos Colosseum

Tales of Xillia: Golden Warrior

Hyperdimension Neptunia Re;Birth2: The Level 999 Colosseum fights.

I found the Star Magician worse as a kid Sliv haha.

I seriously consider Star Magician a really tedious cakewalk.

He's easy to beat without a specific strategy but it takes so looooooooooooong.

I know the feeling. After all the struggles I had fighting the dragons in Bravely Default, I resorted to the ninja method I read about at GameFAQs. It worked with the dragons, but Lord DeRosso still kicked my butt several times, despite my characters levels being in the low 80s and all their job levels at at least 10.
I'm thinking of just continuing the main story and hoping I can finish that subquest later in the game.
It is one I want to finish just because of what DeRosso said he'd reveal if I beat him. Hopefully, I won't have to resort to watching a video of it on YouTube instead.

Info:
Ozma can quickly destroy the party with a variety of heavy attacks, including Curse, Doomsday, Holy, Level 5 Death and Meteor. It will frequently restore itself to full health.

Ozma will not waste a turn using an elemental attack that will be absorbed by all party members (nullifying the element still makes Ozma use it), or use status attacks if all targets are immune to the status. This increases the chance that Ozma will use Meteor.

Ozma has a unique battle script; when Ozma's ATB bar is filling up, i.e. when it is not in the middle of casting an ability, any command the party targets at Ozma will cause its ATB bar to immediately fill up, and Ozma will execute its turn before the player's commands.

Abilities:
Lv5 Death: Inflicts Instant Death on all party members whose levels are a multiple of 5. Ozma will only use Lv5 Death if the party has a member susceptible to it; if no party member is of a suitable level, Ozma will not waste its turn.

Death: Inflicts instant death on one party member if it hits.

Lv4 Holy: Deals Holy damage to all party members whose levels are a multiple of 4. Ozma will only use Lv4 Holy if the party has a member susceptible to it.

Holy: Deals Holy damage to one target. Ozma will not use the ability if the target absorbs it.

Flare: Deals non-elemental damage to one target.

Flare Star: Deals damage based on the characters' level. Can miss.

Doomsday: Deals Shadow damage to all targets, including Ozma.

Meteor: Deals random damage to the entire party, with the possibility of dealing 9999 to all of them.

Curse: Deals non-elemental damage to all party members and inflicts Confuse, Poison, Slow, Mini, and Darkness.

Mini: Inflicts Mini status on all targets.

Esuna: Used if Ozma is afflicted with a status ailment.

MP Absorb: Used if Ozma is low on MP. Drains MP from the party.

Berserk: Used to counter attacks.

Curaga: Used as a counterattack; there is a higher chance of countering with Curaga when Ozma's health is low.

This fucker's fight is so rng-based that before you even get a chance to do your first turn he could've already wiped you out if you're unprepared.

https://youtu.be/QLMcU-6bxsA?t=27m40s