Bloodstained Hands

I can definitely understand why you might be struggling. There are a number of different strategies you can employ for combat, and different strategies may work better in different circumstances (e.g. against singular enemies as opposed to groups). Here are some things you might consider looking into to ease the difficulty:

• Adjust the battle speed and wait modes. If you are still set to the defaults, you may find your slower party members taking less turns and reducing their overall contribution to the party. If you have a bit more of an active wait mode with a slower battle speed, your mage and fighter classes will get more opportunities to act with less penalties, but you may have to plan around queued enemy actions. My personal favourite battle settings are a battle speed of 30% and a wait mode set to Actions.
• Train your spells. If you use the training dummy in the sanctuary, you can level your spells to increase their effectiveness. Spell tomes can be bought in certain cities as well. Equipping a spell tome will grant access to a level one spell, and when used ten times in combat, will level up and become a permanent addition to the users spell list. Some of the tome spells can be invaluable when facing difficult enemies, especially in the late game.
• Grind. As painful as it may be, you might simply be underleveled for the enemies you're facing. Going back to the dungeon you can manage the best and gaining experience and levels may make all the difference.
• Update your equipment. If you have access to a higher tier of weapons and armour, I would strongly recommend taking enough side missions to accumulate the resources to afford the best possible gear.
• Try new things. Your assassins are your fastest characters, but not necessarily your strongest. If you were to dedicate them as support roles, healing allies with items and applying whatever status ailments they can, you may find your survivability dramatically increasing.

If you are looking for specific combat strategies, I'd be more than happy to help give some suggestions, but I encourage you to develop your own style of playing and learn what works best for you. The combat is designed to be challenging, but with the right methods nothing should be impossible. I'd suggest starting with adjusting the battle speed and wait modes. Learning how they work and getting familiar with them can create a world of difference depending on who you're facing.