It takes time to eat so make sure to move around while eating to avoid taking more damage. The longer you eat the more health you get back but you can't attack, if you attack or get hit you will stop eating.
Use health potions if the enemy has a ranged attack to get your life back quicker instead of eating.
It is easier to map weapons to the face buttons and food and potions to the d-pad for tidy and quick access.
Having a melee weapon in one hand and a ranged weapon in the other hand means you are ready for any type of situtation.
Always take out the healers first unless you want to die quick or fight forever.
Grind equipment you dont need anymore to save space and earn cash.
Make sure when fighting to alternate between weapons i.e if you have melee weapons on the A and X shortcuts. First press A followed by X then A again and repeat. You will see an on screen multiplier (x2, x3 etc) to show that you are doing it correctly. Not only does this grant some extra damage but also fills the justice meter a lot faster. if you can chain together 7 attacks and therefore getting the x7 multiplier, the enemy you are fighting will be knocked back.
Play on the hardest difficulty to experience the game how it should be played.