I am not very far in the game yet, but enemies do respawn, so you could just try level grinding for a bit I suppose.
Sorry I can't help you since I'm really not far in the game in offer any strategies.
Are you enjoying the game so far though? Apart from it being hard?
I beat Eternal Sonata about a year ago without too many problems. The last boss is kind of a pain but nothing too bad.
In general I don't remember grinding being very effective. The main thing I remember is selecting the right party members for whatever I was fighting. If you have access to Viola use her! Her arrow attacks are incredible!
Also a huge component of the game is learning to properly block.
Other than that use the same strategies you probably use in other RPGs - focus attacks on single enemies when facing groups. Try and spread out your players to avoid multiple attacks, etc.