The Computer Is a Cheating Bastard - Par for the course in a fighting game, of course, but the computer-controlled characters can do things that are flat-out impossible for human players to do, like equipping ridiculously rare equipment long before the player can craft it, equipping gear for which they haven't reached the level requirement, knowing attacks at levels lower than the player learns them, and they often equip multiple copies of accessories that the player is only allowed to use one of.
A really basic one: the character with the higher level gets about 100 extra BRV points for each level exceeding. Even assuming Player Characters get that bonus (the start-of-battle screen is conveniently set up in a way that you can't find out), computers are far more likely to have the level edge. (Hell, under certain circumstances, enemies will actually break maximum level.)
Chaos is a SNK Boss, so he does a lot of cheating. Zero-start up attacks let him counter your attacks instantly, his HP attacks require precise timing to avoid properly, he has his own customized set of equipment that have a stat boost dependent on his level, ensuring his stats are as high as can be "fair", and he heals fully during the three-battle match, while players begin the next fight with however much HP they managed to retain in the last fight. If the player loses any of the three battles with Chaos, they have to start the match all over again.
Chaos can only be fought in the smallest stage in the game, which has only a ground floor. Chaos's attacks are some of the largest in the game, with wide areas of effect. His attacks take up so much of the stage that it's nearly impossible to dodge, largely because there's nowhere to dodge to.
Chaos' unique summon Shinryu deserves a mention too. Normally, players can call a summon once per battle. Shinryu can be summoned as many times as Chaos wants to use him, and can choose his effect, because Shinryu mimics the effects of other summons and powers them up. If Chaos really wanted to he could summon Shinryu three times in a row, activating an effect to cut your Brave in half every two seconds, doubling his own Brave instantly, and then lock his Brave so your attacks can't damage him.
The battle with Chaos is a bit easier in Duodecim: if you die on the second or third phase of the battle you start over from that phase, and you can have up to five characters in your party so if one dies you can continue with the next and Chaos will retain whatever HP damage the defeated character did to him.
There is one occurrence of "justified" cheating, if that makes sense. The Laguna ghost the player fights is a Squall in the Level 20 range, equipped with Level 20 armor, but a level 94 weapon. This is justified because the weapon is a Machine Gun, Laguna's signature weapon in Final Fantasy VIII. Its still technically cheating, but there's an actual reason in this case.
In Dissidia 012, special Emblems on the boards trigger certain effects in battle, like increasing your Bravery, making Boosters stronger, increasing the critical hit rate of certain attacks, and so forth. In the "Confessions of the Creator" storyline, enemies are often positioned in such a way to force you on an Emblem with a detrimental effect, like disabling your equipment, starting you with no Bravery, and making physical or magical attacks do no damage. This is in addition to the other cheating described above - these mentioned enemies that you must face with no equipment will be over level 100 with top-grade equipment and accessories. Oh, and with most of the Emblems, at least the detrimental one, only you suffer these drawbacks - should the enemy be positioned on an Emblem, it won't take effect on them.
Well, you could use a skill chain to avoid standing on them. Although you might be standing on a different bad one, so it's bad for you either way.
