Some battle strategies that worked for me when I was playing the game, and that made the battles less difficult: Generally, ranged weapons and spells work better in this game than melee weapons. So be sure to have a mage in your party, especially for the later stages of the game, and have spare bows and arrows/bolts for every character.
Cast Stinking Cloud on the enemies. It is an extremely handy spell, that will knock most enemies unconscious for some time. Kill them with magic and ranged weapons, but be careful not to step into the cloud yourself, so don't use melee weapons.
Another very good spell is Monster Summon. If you summon monsters, however weak they might be, the enemies' attention will be drawn towards them. While the enemies are busy dealing with your monsters, wasting their best spells and attacks on them, you can gain time and prepare yourself for the actual fight.
You'll often have to fight not just one monster, but a party such as your own, usually including fighters, mages, etc. The best strategy in such a case is to put your party in a safe place, then approach the enemies with only one character. The moment you spot the enemy, run back. The fighter of the enemy party will usually follow you, leaving all the others behind. Just bring him to your party and kill him easily with your six people. Then heal and rest, if you wish, and repeat the procedure until all the fighters in the enemy party are eliminated. Killing the remaining mages or whatever is left from the opponent party shouldn't be then so difficult. This strategy worked for me in most difficult battles, including the final one.