For what it's worth, I think the boss battles can take up some significant amount of time as it is, let alone the cutscenes and regular battling. I wouldn't say I did much extra training before the final battle. I thought I'd just give it a go, see how it went - and somehow won them all. Fighting buttloads of enemies earlier in the game probably helped, too, but I didn't do any more than about eleven marks before moving on to chapter 12 so I'm not sure.