One of the basics of survival in any combat is to kill the bad guys. There are nearly limitless ways of accomplishing this task, and the less work that you have to put into it the better.
The first key to keep in mind is that the bad guys to in fact damage each other. One of the greater fears about spell casters and their ilk are area attacks that can take out the entire party at once. Often these types are paired with minions or partners whose job it is to bunch the party together.
Your strategy in these situation should be to always make sure that as many enemies as possible are within all possible areas of effect. In other words if the caster wants to hit any two party members they should also be forced to hit at least one of their own allies. This is most easily accomplished by keeping an enemy between the party members.
Next, look for opportunities to force the monsters to use hazardous terrain or to provoke opportunity attacks. If you have a decent rogue type along then few traps should surprise the party. Knowing where to avoid makes combat not only less dangerous but can be used to your advantage. you can always intimidate and bluff to good effect. Also, using ranged attacks and blocking the good spots with skillful fighters can force enemies to either risk hazardous terrain or opportunity attacks or be picked apart mercilessly at a distance.
Finally, never forget about compulsion effects. They are a great way of taking control of an encounter. These can be very powerful effects, starting at with charm person. If you do manage to take control of the enemy, abuse the power mercilessly. Lesser effects such as suggestion and charm person won't allow you to cause an enemy to attack an ally or damage himself, but can be used to maneuver them into more vulnerable positions or to trigger traps of which they are not aware.
Always keep in mind that the outcome can be swayed very quickly by using strategy and good judgment.
Kavorak HighCastle Fighter