I have noticed in my game I'm doing things that the enemies are
doing to me.I have used Kelsey with stoneskins to occupy fighters
while the others concentrate on finishing one opponent.
So here are things I use and a lot I learned from reading.
1.Always read the text.It tells of immunities,spell defenses and other
stuff.I now read mages defenses and see the weaknesses there.
2.Run and use the walls.Bottle necking wins tons of fights.Sometimes
the enemies split up and don't know what to do aginst you.
3.Distractions.Including summons and mages while they breach.
I use Kelsy buffed with defences while he casts spells to occupy
enemies that can't hit him.
He can't hurt them with spells either because most though enemies
are resistent to magic and immune to fire,acid,cold.
4.Breach everyone that is immune to physical damage.
5.Mages can be killed with melee when they don't cast protection from
magical weapons.
6.Have weapons that do crushing damage.Haste and everyone on one golem
at a time.I use F/M or sorcerer that instead of hiding behind unable
to hit a golem as a shield for Keldorn or Minsc.