Use active pause. Change game settings so that each encounter starts with active pause. Use it as often as you need. You're not capable of giving orders to six characters at the same time.
Be careful with some abilities. Especially offensive spells with area of effect. It might happen that you will not only kill your enemies, but your whole team as well. Even shooters can hard their companions when they stand in their way.
Position your party in the right formation. Characters that use melee combat should stand in first line, shooters and mages behind them.
In bestiary you will find information about all encountered enemies. The more monsters you slay, the more information you will gather.
Make use of your surroundings. By blocking a door with warrior, you make it impossible for enemies to get to your shooters.
Use support items. During harder encounters don't be afraid of using potions or scrolls.
Enemy is too strong? Return to him later. Sometimes You will meet enemy that you have no way of defeating at current level. Return to him after you gain some experience.
Frequently study the bestiary. There you will find valuable information about encountered creatures. This knowledge might be useful when facing stronger opponents.
Always have two weapon types. Sometimes enemy is resistant to a certain weapon type, it is a good idea to change your weapon to another one in that case.