I cheesed most of the game by spamming Nazuna's sakura storm,paired up with her passive Zanshin (10% sp restoration after every battle), and she has that combo once she hits level 20. Paired up with gears to increase her crit rate and atk, she made the entire game a breeze.
There was only three points in the game where i actually ran into difficulty and had to re-think my strategy. The final boss of the main story,where getting immunity to certain statuses is key (forgetful and stun), The emperess of rage (Scalnaeda's descendant) because i was underprepared, and underleveled, and the very final secret boss of Gramtoa.
For the last optional boss of gramtoa, you need to be fully upgraded,and you cannot hold anything back. The boss is absurdly hard and makes every single boss before it a joke.
To beat it,i followed one of the guides the comments posted on the japanese walkthrough page.