You don't do preparation before every fight? How do you kill all those enemies in a single day?
I just killed them.
Protect DPS carries and move melee fighters forward. There were only two fights that were difficult for my party and spider ain't one.
I had wizard as my main character, Lai'zel, Astarion and Shadowheart. Magic missiles + fire bolts from my main. Healing from cleric, and sneaky stabs from Astarion done magic. Lai'zel most of the times hit for 12 - 15 dmg.
Also move your melee units to the side of back of your enemy. You will have much higher chance of successful hit. (Forward attacks are more dodgeable)