Final Fantasy X/Secrets and Sidequests/Monster Arena/Original
The original creations are unlocked by collecting various combinations of fiends (e.g. one of every fiend). Hitting for 99 999 with quick hits is a necessity at this point as they are the hardest creations you will find in the monster arena, but are still a hell of a lot easier than the dark aeons.
Each of the below fiends will cost 15 000 gil to fight with the exception of Nemesis which costs 25 000 gil.
 Earth Eater
If you intend to max out your luck stat (or even increase it any considerable amount) Earth Eater is almost certainly going to quickly become your most hated enemy in the game. Earth Eater drops 2 fortune spheres when overkilled (or dark matter as a rare drop if you're unlucky), making him the only way to get more than about 5 fortune spheres. Of course, compared to raising your other stats you will need at least 3 times as many fortune spheres as you need a set for each character whose luck you wish to increase. It should be noted that Earth Eater is not a hard battle, just a long one (as with a lot of the original creations).
At first when Earth Eater is standing he will counter everything with Megaton Punch which inflicts instant death and a large amount of damage. This shouldn't be a problem as long as you have Auto-Phoenix, coupled with the fact after 2 hits at 99 999 Earth Eater will fall over and no longer be able to use Megaton Punch. Earth Eater will have just under 1 million HP remaining at this point, and won't be able to attack you on his turns. Therefore, if you are sufficiently powerful, you will be able to finish off Earth Eater before he gets back up.
Of course this wouldn't be an Original creation if it were that easy, so when on his back Earth Eater counters everything with Flare (which he reflects off himself) which hits for a maximum of 9 999 damage. With a low magic defence this may well kill you, so auto-phoenix is your best bet. At higher levels you may find shell helpful. After a large number of flare animations Earth Eater will finally turn to Pyreflies.
 Greater Sphere
Th'uban looks much like the Sanctuary Keeper from Mount Gagazet, but you'll find a substantial difference in difficulty.
Anima and the Magus Sisters can be powerful tools in this battle. If you've been training Yuna lots, you'll find that the majority of attacks to these two aeons will be reduced to around, or even below 1000. It's a good idea to start the battle by summoning Anima and casting Oblivion, followed by as many counts of Pain as you can get in before you need to recover. If Anima should die, make sure that Yuna can summon the Magus Sisters quickly before Th'uban can attack your party, as he'll do substantially more than he did against your aeon (Condemn can wipe out your party in one, if your defense stat is low and you aren't into the 10,000s with HP).
When the Magus Sisters are summmoned, you should be able to win the battle very easily. "Fight" or "Go, go!" willl usually be your best option, but use their healing options whenever you need to. When their overdives are full, providing Th'uban hasn't cursed you, you can use the sisters' overdrive, and unleash a combo of 99999s, which will usually finish him. Don't worry if you can't use the overdrive, though - You'll still be able to attack it, and deal a substantial amount of damage.
This battle is even more tedious than those with Jumbo Flan and Earth Eater with the sole redemption being that you only have to do it once. This newly designed fiend (the only one in all the monster arena) has a massive 4 million HP and 3 distinct stages. It can attack (sit on) a single character for a most likely fatal amount of damage, use Megaton which inflicts a large amount of damage and confusion, or worst of all slime which can hit one person or the whole party for a fairly large amount of damage and a variety of fruity status effects (armour and mental break, curse, poison and delay). It can also cast Curaga and Regen. If you are having trouble with slime, it isn't difficult to silence Neslug which pretty much restricts it to using Megaton or his normal attack.
At first just use quick hits until you've done 2 million damage and Neslug retreats into his shell (you should be able to do this before he even gets a go, if your agility is over 150 or so and you have auto-haste). At this point Neslug will no longer attack you, instead using Curaga on himself for around 8 000. Far worse than this he will regenerate upwards of 15 000 HP every single turn, and becomes immune to physical attacks. He will only come out of his shell if he replenishes the 2 million you've already taken off, or you take off a further 1 million HP which shatters his shell.
You need to ensure you do a lot more damage than the HP it heals every turn, and summoning an aeon is not wise either as he seems to get his HP back 10 times as fast (over 150 000 a turn!) if you do this. The only way to inflict 1 million damage in the shortest possible time period therefore, is to doublecast Ultima and then copycat it, or use Blitz Ace from Tidus, then keep Entrusting your overdrives to him. Three overdrives should do the trick and bust the shell. At around 160 magic each Ultima will inflict about 50 000 damage, so against his healing this means you will take off around 80 000 a turn. Doublecasting Ultima takes forever, and gets very old after seeing it the 30 or so times you need to before moving Neslug to his final stage (this will take well over 5 minutes).
After this extremely tedious process, Neslug's shell will finally shatter and you can use quick hits to knock off the final 1 million HP and squash this seriously overgrown mollusc.
 Ultima Buster
It takes one hit to kill the head which will stop Ultima Buster's most deadly attack. It then takes one hit to kill of each the arms, which should be killed as soon as they come back. The main body of Ultima Buster has 5 million HP, but doesn't counter anything, making this battle surprisingly easy. Once you have killed the head proceed to just using quick hits on the body until it dies. On Ultima Buster's turn he will either use a single physical attack, which depending on your defence and HP you may survive, or use Ultima. If your magic defence is low you may need to call an Aeon to block Ultima Buster's turns, or make sure to have Auto-Life on one of your characters, but other than that this is actually one of the easiest original creations. If you can easily defeat him, Ultima Buster drops weapons with Triple AP, Triple Overdrive and Overdrive -> AP on them. If you don't have them already, these weapons are obviously fantastic for gaining sphere levels, but they also sell for a few hundred thousand gil which makes your job of maxing out the sphere grid a little easier.
This is difficult, so you should have 255 in at least strength, accuracy and agility (you can get strength spheres by killing the juggernaut). You'll also need Quick Hit to finish him off as quickly as possible. For weapons, use the fully upgraded celestial weapons (or any weapons that have break damage limit). For armor, try to get as close to the following as you can: Auto-Shell, Auto-Protect, Auto-Haste, and Auto-Pheonix. You can get four-slotted items from O'aka in the Macalania Forest, at the junction between there and the Thunder Plains. Now, if you don't have those abilities in your armor, it's okay, you can survive without them. Just make sure everyone in your party knows Auto-Life, and you have at least 4 Mega-Pheonix's. You should have Auto-Life on (preferably) one person at all times. This way, when Nemesis uses either Armageddon or Ultra-Spark, he'll be revived, and you can use a Mega-Pheonix.
Whatever you do, do not attack with anything besides Quick Hit, since he counters Attacks, Overdrives, Black/White Magic and items with Ultima, which hurts. You should use Quick Hit since he is immune to all status ailments, and skills are the only things he won't counter, and you can hit repeatedly a heck of a lot more quickly with Quick Hit. Now, just unload everything you can on him, hit after hit. It should take around 15-30 minutes for the person with average stats. If you have 255 in all stats (besides luck), it should take about 10 minutes, but that's only if you're willing to work for it. Make sure at least one person has Auto-Life, and everyone has Auto-Pheonix, otherwise the battle could be lengthened to around 45 minutes or more. Not fun, at all. If you can survive his Ultra-Spark ability, watch out, because it gives you status ailments, including poison.
The key to this entire battle is having one character with Auto-Life, lots of Mega-Pheonix's if you're not all that strong, but on average around 4-5, and everyone having Auto-Pheonix in their armor. Don't use overdrives, regular attacks, black/white magic (on him, it's okay to use it on yourself), specials, or items (once again, okay to use on yourself, just not on him. Don't forget to grab Nova from him for Kimarhi. In case you don't know, Nova is Kimarhi's ultimate overdrive. If you want to see it, just Lancet him and run, get Kimarhi's overdrive and use it, or fight Nemesis and wait for him to use Armageddon. Same thing, different name.