Final Fantasy X/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.

Neslug
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. 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, 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.