Which are supposed to be harder?

[RAWRmonger]

Archeoaevis (spelling?)
Hell Wyrm
Zodiark

Because I just killed Hell Wyrm.
Then died instantly to archeoaevis.
And I'm stuck in Zodiark's final health segment, although admitedly, I'm using brute force strategy.

Also, is Omega Mark XII hard? I still have to go get hastega for the achievement and then I wanna bail out from that ♥♥♥♥♥♥♥♥ dungeon.

[arenaynieve]

Ugggghhh..... Hastega... the bane of my existence lol. I hate the great crystal with strength, with wrath, with passion. It should have never existed X_X Omega is... tricky maybe? He (umm... even if its a robot or something) attacks fast. In the PS2 i had to planify my gambits carefully. In this version I am not sure actually. Also... I dont wish to spoil it because where´s the fun in that :P?

Edit: from those 3 buddies... I dont remember Archeoavis. So I´d say Zodiark. I reckon he can be a ♥♥♥♥♥ if you dont hurry. You really want to finish him off with a nice quickening chain when you have the chance.

[RAWRmonger]

I beat Zodiark a while ago, with a strategy this time (beserk, bravery, protect, shell, haste and with exclaibur, holy lance and a white mage, then finished it with scathe motes).
Only missing Archeoavis (hardest pricks on Zertinan Caverns), Yiazmat (only went to check it out, then bailed), Omega mk12 and Rare Games (dunno if they're strong).

[Hinnyuu]

None of them are particularly difficult if you're using the right strategy.

Archaeaevis, while certainly powerful, are probably the weakest of the three simply because they're so easy to cheese. You can fight them at a zone line and dodge all their Curse or Dispelga casts. They are also susceptible to Blind as well as Confuse, meaning you can slaughter them with Karkata (which is available as early as Henne Mines/Eruyt Village).

Hell Wyrm is mostly a joke. It's not hard to defeat even without ever using Reverse of Wither, and since it's weak to Holy you can bash its head in with Excalibur right quick. Still a long fight, of course, but not really dangerous in any way.

Zodiark is, all things considered, probably the "hardest" optional boss, simply because the physical immunity towards the end (plus the usual damage reduction) and the random deaths to Darkja are major annoyances. That's not to say you can't butcher it in a blink, of course, since Holy weakness once again means your righteous meat cleaver Excalibur can go to town. I believe it's possible to kill it so quickly it doesn't even have time to put up the Paling, but I haven't personally done it.

[Schloss Ritter]

Biggest trick to Zodiark is to not use Dispel like you would against any other boss, as this triggers his AI to use Magick Barrier later when the Palings go up, making him almost invulnerable.

[Hinnyuu]

Originally posted by Schloss Ritter:

Biggest trick to Zodiark is to not use Dispel like you would against any other boss, as this triggers his AI to use Magick Barrier later when the Palings go up, making him almost invulnerable.

Literally invulnerable, but luckily only for 2 minutes. Still, it probably is better not to dispel.

[Cinnamon Tee]

I'd say that Hell Wyrm is the easiest of those three. It's damage isn't anything to worry about even without breaks, and with Excalibur/Holy Lance+bravery it goes down relatively quickly even with it's huge HP pool. Unlike most of the other optional bosses in the game it doesn't have any special gimmicks like field effects, so all you really need to worry about is it's relatively high strength and the Stop status inflicted by it's special.
Archeoavis is more annoying due to Curse, but with Ribbons and a Karkata it's just a matter of surviving long enough to get a few Withers off.

Zodiark is generally the most difficult of the three due to the luck-based nature of the fight. There's no getting around the Death effect of Darkja, but you can minimize how often it uses Darkja. If none of your party is vulnerable to dark element, it will replace Banish Ray with Darkja, so make sure to always leave someone vulnerable. And as has been mentioned above, don't Dispel it at all during the fight, which unfortunately means you'll need Opal Rings on your mages to deal with Reflect.

Omega is a rather simple boss that really is only difficult at the start. It uses only one attack that at full strength will always kill a character in two hits. But, that attack is based on his Strength so a few successful Withers renders it almost completely harmless.