Feck, I've just learnt that I should have talked to the kid in chapter 14 before going up to the Shinra tower and now i don't think I can get the leviathan battle. I was so sure I did everything before heading up. I'm up to the point now where I've just saved Red 13 from Hojo. This is probably the first chapter where I think they might have drifted a bit too far from the original. It's getting kind of messy. I'm still having fun but i'm concerned where it's heading
Depending how far you've gone you should still be able to talk to Chadley in Hojo's Lab (the bit where RedXIII jumped through). The battle with Leviathen is unlocked after you've done a certain intel mission. But it's only a minor thing and as Raw says you can always load up and get it when you've finished the game.