Thanks BlazeBreak and WesternHeretic.
I've beaten the game today, wooo (Normal mode). Managed to kill Bhunivelze on my second try w/o any special preparations meaning I used the same setup of Dresses, Weapons, Bracers, Abilities etc. as in the last 20 hours of game. I did not use Preta Hood/ The Other One for nullifying any kind of damage either. I didn't want to bother changing my setup at all... I got used to my 3 Dresses (Velvet Bouncer, Cybersomething and a beautiful Red One) and the Pumpkin Head
Oh and I rushed through the Cathedral before - didn't do the Mog's quest as well. The difficulty spike in the last dungeon is pretty intense, lol.
On the first try I barely managed to get to his 4th and final form - I used almost all the Ethers and my only Elixir on the 3 forms before. Of course I could not stagger him (Thundara 2, Aerora 4) for the life of me and so he killed me pretty easily.
On the second try I managed to kill the first two forms just by spamming Heavy Slashes - no Staggering, no Overclocks etc. just one X-Potion. The 3rd form is where I first used Overclock and 1 Ether. When the final battle started I spammed Overclock + Ethers just to Stagger that hard-ass motherfucker. It worked and I was on my last Ether!
And man... it was totally worth for that kick-ass CGI ending, wow. It was incredible. Terrific! I love Bhunivelze's design and that one scene on the planet below when you can see his gigantic...corpse? shell? and Caius/ Yuel in front of it - whoa. I wish to had that picture as a Full HD wallpaper. Great final boss for the whole Trilogy and a very, very impressive ending. I was totally not disappointed unlike with the FF13-2 ending, heh.
I still need to kill Aeronite and Ehrzegkiel (sp?). The second one - without any kind of preparations - I managed to get down to 1M HP (out of 7M) before he insta-killed me with some spell for 32 000 damage (!!!). Had I two Ethers more I would've kill that son of a bitch - and I dumped away one Ether before because of lack of space, damn it. Still - the Ultimate Lair Boss - is 10x times easier than Aeronite.
Now to start the NG+ in Hard or Easy mode? Decisions...