When playing through FFX-2 for the first time in many years, I was at first dismissive of the Creature Creator system, thinking it merely a goofy addon. Then I discovered the Fiend Arena, which eventually pits you against numerous monsters so appallingly strong they put even Trema to shame. Then I looked around on the net for info - how you can possibly beat these insane creatures, and while specific information is available, I found none of the in depth details I'm used to having with older FF games. There are plenty of suggestions as to abilities to grant your creatures to maximize their power, but very little describing how to do so.

Update 2022, Apr 9th: I have checked the effects of all items, accessories, garment grids and dresspheres in game, and reported what I found here. This guide is no longer reliant on external sources for rare stuff. Also tweaked some text here and there, and separated out the accessories section in a more logical manner.

To get the most out of your fiends, you'll need to make use of the Creature submenu, found near the bottom of the main menu. Analogous to the Customize menu in FFX, this menu allows you to consume items and accessories in order to grant your Fiends specific commands or auto-abilities, as well as purchase direct stat buffs. You can also have a Fiend unlearn unwanted commands and auto-abilities.

Each fiend can have up to 4 innately known Commands and 4 innate Auto-Abilities. There are 8 slots for each shown, but 4 of these slots are reserved for equipment based abilities - one from a Dressphere, one from a Garment Grid, and 1 for each accessory slot. This means it is relatively easy to suppress an unwanted natural ability (such as the Cactuar's 1000 Needles) by having it forget that ability and then ensuring it knows 4 other abilities.

I won't provide specific strategies for you - this is meant to be a general list of effects, not a strategy guide.

Note that when adding HP or MP while a creature is equipped with things that add a percentage gain of that stat (Iron Bangle, etc), it will appear to gain more HP than it would at base stats, as pointed out in the comments. After some testing, this appears to be because it adds the same amount of HP to the base HP, which is then being increased by the percentage bonus of the equipped accessories and garment grids, rather than an effect that will actually result in a more efficient gain in these stats by first equipping bonuses to those stats.

Note that these are effects for consuming accessories in the Creature submenu. Equipping accessories will have the same effect you're familiar with from the main party.

Garment Grids have a very different effect for captured fiends when equipped than they do for the main party. Each Grid has an associated Command, Autoability, or both that is usable when that Garment Grid is equipped, and several provide a statistic boost as well. As fiends cannot spherechange, there are no gate effects as there are for the main party.

Commands marked with a star (*) are only available to fiends with access to the Attack command, though you may still use these abilities if you have Unlearned Attack.

Like Garment Grids, Dresspheres provide access to a specific Command, Auto-ability, or both. This limitation on commands is actually a good thing, as the AI often makes poor decisions when it has access to a wide range of commands, like those available to the main party when using a mastered dressphere. I have not included the stat bonuses here, as the effects of dresspheres appear to have much more variability than they do for other equipment. Just keep in mind what the strengths and weaknesses of each dressphere were in terms of stats, and remember that there are ways of boosting stats that you find are too low using items and accessories.

Commands marked with a star (*) are only available to fiends with access to the Attack command, though you may still use these abilities if you have Unlearned Attack.