Final Fantasy X/Secrets and Sidequests/Monster Arena/Area Conquest
The area conquest creations are unlocked by collecting one of each fiend in its respective area. They are the easiest creations you will find in the monster arena, but if you already tried to fight Chimerageist you'll realise they're still a lot more powerful than anything on the main storyline.
Each of the below fiends will cost 6000 gil to fight with the exception of Don Tonberry which costs 8000 gil. As you might expect as you go down the list each creation in general is harder, apart from Chimerageist who is the easiest monster arena creation (as he is the first one you must unlock).
There's very little to say about this overgrown bird, other than the fact that it is basically a beefed up Zu. It has 320 000 HP and will drop down to the ground when you've knocked it down to about 100 000. When in the air it will attack you normally or attack your entire party for relatively large damage after its wings glow. Upon dropping to the ground it will lose this second attack. At low levels Aeons should easily be able to take this fiend down (eg. Anima or the Magus Sisters, possibly even Yojimbo due to his high evasion stat) or failing that try boosting your evasion.
 Malboro Menace
As if Great Malboros weren't bad enough, this is the big daddy. It has 640 000 HP, and no surprises for guessing that it will always ambush you and will always use Putrid Breath on that first attack. Putrid Breath only differs from Bad Breath in that it does about 1000-2000 points of damage on top of all the status effects. There are three ways of dealing with this:
- Have a weapon with First Strike on Wakka, and if he can execute a perfect Attack Reels at any more than about 55 000 a hit it will die (with a nice Overkill).
- Have a weapon with First Strike, switch Yuna in on the first turn, and summon an Aeon to soak up Putrid Breath.
- Have an armour with Berserkproof, Confuseproof and Auto-Med on at least one person, or more ideally, Ribbon.
Putrid Breath may come at any time when it gets a turn so the last option is probably the best one (although the most time-consuming to make available). Its other attacks consist of Chow Time which at lower levels will kill who it hits (it does upwards of 12 000 damage), a slime attack which can do about 6 000 to one person, and Mega Gastric Juice which is similar to its slime attack except it hits everyone for a few status effects and about 5 000 damage. High-powered aeons like the Magus Sisters at a decent enough level will take care of the Malboro Menace for you, or appropriate status-resisting armour will take away the only really threatening aspect of fighting this crime against nature.
Kottos counters everything, which at low levels will most likely kill you, so as long as you have auto-phoenix you should be fine as he has no other multi-hit attacks. He has 440 000 HP which may take a while to knock off with all the counters but you'll get there eventually. Fighting Kottos is useful even later in the game as overkilling him after using a Distil attack or a Distiller from the 'Use' command will make him drop 40 of the relevant sphere (20 if you don't make the overkill which is around 15 000).
Coeurlregina is another relatively easy creation, although a significant step up from Chimerageist. It has 380 000 HP and although has multiple attacks the most commonly-used one is Chaos, which inflicts Confusion, Curse and Doom. The only problematic one in there is Confusion which is blocked by Ribbon or Confuseproof, and any character with Auto-Phoenix will revive the Doomed character when they fall. To supplement this, it also has Hyper Blaster which is instant death to one character (immediately countered by Auto-Phoenix), and a handful of black magic spells like Drain, Osmose, Flare, Blizzaga and Thundaga. Really, Chaos and Hyper Blaster are the only potentially-problematic attacks it has; at 99 999 a hit Coeurlregina will go down in four attacks, and aeons are immune to everything except Curse, but that won't matter with the Magus Sisters, as their normal attacks are more than powerful enough to take it down. Also, Coeurlregina has no multi-target attacks, so two characters with Auto-Phoenix will mean you physically cannot lose this battle whilst you have Phoenix Downs.
Jormungand's creature model is pretty much identical to that of the Anacondaurs in the Calm Lands, so you should guess that this thing might try and petrify you. Although Stoneproof on everyone is all you really need for this fight, Auto-Shell on everyone helps as well as its tail attack which hits everyone can hurt quite a bit. However, its quite slow, and has 520 000 HP, meaning six hits at 99 999 kills it which is easy to rack up before he gets a turn with Quick Hit and a decent Agility stat.
 Cactuar King
The Cactuar King only has 100 000 HP but has completely ridiculous Agility and Evasion stats. Most of your attacks will miss, and it counters everything (regardless of whether the attack connects or not) with 10 000 Needles which does, as the name suggests, 10 000 damage to one person. On its turns, it might use 10 000 Needles, or even better, 99 999 Needles which is inescapable instant death to one person (not because it inflicts instant death, but because 99 999 is the theoretical maximum amount of health you can get, and even at that it'll still kill you). Sometimes (10% chance), it might even run away! Wakka is probably the only character with a high-enough Accuracy stat to actually hit the Cactuar King reliably, but rejoice in the fact that two hits at 50 000 minimum will kill it, or an Attack Reels/Blitz Ace will sort it out.
Espada is one of those blade-spider type enemies, and as ever has an annoying status effect on its attack. Unfortunately this status effect is death, and its attack is used as a counter. At first this sounds like it will be a tricky fight, but either Deathproof or Auto-Phoenix will see you through hacking down the paltry 280 000 HP or in more desparate times using Zombie Attack on your party members on an Aeon will ensure your victory too.
 Abyss Worm
The abyss worm has low defense and (relatively) low HP and strength. There are no problematic attacks and its agility is pathetic. Sometimes it may swallow a party member when it gets low on HP, and spit them out again on its turn but this will once again cause no real problems. The only thing to worry about is earthquake, but this will only be a problem at very low levels (in which case you should be using aeons). Therefore whittling down its 480 000 HP should be a doddle.
The easiest out of all the creations in the Monster Arena, and also the first one you unlock. He has 140 000 HP and the same attacks as normal Chimera fiends, except they all do a lot more damage. At 99 999 a hit it'll go down in two attacks with an Overkill, but you'll be able to beat this guy with much lower stats than are necessary to accomplish that; a moderately-powerful Yuna (even before the end of the game) can summon the Magus Sisters which will tear Chimerageist apart - Sandy's Razzia will pretty much always hit for 99 999 at this point, and the other two will do a good job of inflicting damage and keeping everyone alive as well. Its physical attack and Megiddo Flame might hurt a little, but the former can be reduced with Protect and the latter with NulBlaze or absorbed with Fire Eater. Thundaga and Blizzaga won't hurt much at all in comparison, and neither will Aqua Breath, the only difference being it hitting everyone and not just one person. All of these can also be nulled (using the appropriate Nul- spell) or absorbed with the appropriate 'Eater' ability.
 Don Tonberry
The Don has high defense and strength, and has a very powerful Karma attack that will damage you based on the number of fiends you've killed (and probably kill you). Let him walk towards you for the first 4 turns and he will no longer use his Karma counter, at which point you can proceed in beating him to a pulp with quick hits. As he only attacks one character at a time you'll be guarenteed a win if you have Auto-Phoenix, although at lower strength levels it may take a while to deplete his 480 000 HP. As a side note, you can gain ludicrous amounts of AP by exploting the Don's Karma attack as long as you have Overdrive -> AP and comrade overdrive mode (see the Monster Arena page for full details).
Catoblepas is basically a super-powerful Behemoth. It has 550 000 HP and all of its physical attacks can hurt quite a bit, but it has a weak Flare spell as well. Protect and Shell will definitely help in this battle, as will Hastega for allowing lots of turns. The only problem introduces itself when you actually kill it; just as the Behemoth King cast Meteor upon death, which may very well have meant a sour Game Over the first time you fought them, Catoblepas will cast Ultima as it dies. This can be reduced by Shell, but will still do quite a bit of damage. Casting Auto-Life on one person before you kill it might be a good idea in case Ultima does more damage than you expect.
Abaddon has 380 000 HP, which is quite low, however if your Monster Arena strategy consisted of using any attack spell other than Ultima, then you're out of luck here because it has constant Nul-All and Reflect. It's also armoured, so your attacks won't do as much as they would normally, but with a decent Strength stat this will not matter. You can speed this battle up with Attack Reels or Blitz Ace, possibly even finishing it in one turn. Its repertoire of attacks consist mainly of magic, with Pharoah's Curse (as with Demonoliths) and Emblem of the Cosmos, which it will use after a 'charge' attack, which is a fairly weak magic attack to everyone (can be reduced further with Shell). You can cast Reflect if you're in trouble to bounce his feeble spells back at him, although if he casts -aga spells it won't do anything.
Vorban is the Barbatos from Inside Sin except with a truckload more health (630 000 HP). Unlike the Barbatos, Vorban is immune to Armour Break so you are going to have to do without it. Repeated use of Quick Hit, Attack Reels and Blitz Ace will speed this battle up. None of his attacks are particularly dangerous, except maybe Mortar which he will randomly counter any attack with (he may or may not counter), and it does a small amount of damage to everyone.