Aciel is one of few bosses that isn't much more challenging on Hard Mode than on Normal. He still qualifies for That One Boss however, because of his Sol Niger attack. It reduces your entire party's health to 1 and can not be dodged. The good news is that because of the fact that Sol Niger makes Defense effectively moot, you don't have to worry about buffing defense and can just focus on burning his health down once your Agility is up. He does have more attacks, but they're not much of a threat, even on Hard Mode.
On that note, Agility is all but required for this fight, as he always follows up Sol Niger with an attack. Some people simply rely on reviving and re-summoning party members, but that's extremely risky. It wastes lots of press turns and drags the battle on longer, creating more potential for him to kill the demi-fiend, which you obviously don't want.
Albion and Skadi
For the sake of thoroughness, I'll give you guys pointers on these 2 as well. For Albion, there really isn't much to tell, aside from the fact that the big guy and little guys revive one another. You only need to kill one of the little guys with the big guy though, you can kill off the other 3 beforehand. You do need to know their hit point totals, but that information is available on various websites like gamefaqs and the Mega Ten wiki.
Skadi isn't hard, but you may wanna focus more on Defense than Agility, as she has a powerful attack that isn't easily dodged, even with high Agility. Other than that, she's not too hard.