The final boss is all kinds of insane.

I just finished File 3... and everything just clicked for me. What. A. Game...

Although the camera still likes to fuck with me xD

Just finished file 4 and so far I love this game.. I I've been playing on casual but I'm thinking about upping the difficulty.

I bumped up to PT.  Casual was too easy.  PT has decent challenge but it isn't crazy.

BTW... how does the ranking for each mission works? I mean... I thought that health or using items would affect my score... but I had battles were I literally lost a battery amd I used like two or three medicines and I got an A xD

It's points based each rank has a threshold that you need to hit to obtain it damage taken isn't penalized, what builds up points are using a variety of attacks in battle such as Sync attacks, chain binds etc... so the more abilities and such you unlock and equip the more points you can potentially rack up.

I see... Well, I like it this way. I was worried that I would get D or C because of damage... now I only have to worry about playing the best I can without fear of using items.. Other games from Platinum like TW101 and Bayonetta really punish you for using items...

Well ... it's definitely from the Nier team, in that regard  

yeh I think I will also it's just very easy I haven't came close to dying once on casual. 

I started out on casual and had to change.  It is far too easy on casual.  I still haven't died on PT, but I at least get knocked down occasionally.