The best defense is to simply never get targeted with an attack.
Fly and Improved invisibility will cover 90% of your defensive needs. Nondetection is also a nice add on to this combo, and with extend spell you can literally cast it a day ahead of time.
Ghostform is also very powerfull defensively.
For a great offensive tactic, take silent spell and summon monsters while invisible. You don't even need improved invisibility for this, so you can theoretically go through a whole dungeon on just one invisibility spell. Even a summon monster I spell can be usefull to provide a flanking bonus to a fighter, and absorb an attack from a monster, so this is really a pretty good tactic> It will shut up your critics.
Enchantments are also good offensive spells for you, because they:
A: Have no visible effect, so the bad guys don't know you are even there.
B: They allow you to turn bad guys into meat puppets so that they can fight on your side.
Confusion, Dominate Person, Charm monster and hold spells are all good fodder for you. silent spell and improved invisibility will make you undetectable to all but the other spellcasters you fight.
When using these tactics, save a few silent spells for close quarters. In larger rooms/outside, simply cast a non-silent offensive spell, and move. Since you are invisible, the enemy won't know what square to attack. Your flight gives you 60' of movement, so you can range far and wide with this. Note that this doesn't work with monster summoning, but it works just fine with most other spells.
Against spellcasters, you really want to use your abjurations. Energy resistance, spell turning, dispell magic/counterspelling are all good choices. Of course, having a good miss chance (displacement + blinking actually stacks in this case) and a good touch AC is the best defense against ray spells and such.
Casting round after round of defensive spells in the middle of combat is unnecessary, booring and really letting your party down. A better tactic is to use extend spell and persistant spell to cast a bunch of defensive spells ahead of time. Preferably the night BEFORE you enter the dungeon. Then, in combat you already have all your defensive spells up. You can even cast spells at bedtime and memorize new spells in the morning, allowing you to actually restock your spell slots with offensive spells, and still having all your defenses up. This is a really uber tactic, and is totally legal, so USE IT.
In combat, you should cast only one defensive buff on the first round. Cast two only in extreme circumstances. Spend the rest of the combat attacking and, more importantly, controlling the battlefield. this will protect both you AND your party far better than another defensive spell. A well-placed wall of stone can end a fight completely. Shrunk items can be extremely usefull here. Again you can pre-cast this spell days in advance and have plenty of 10' bonfires, wooden or even iron walls, and 55 gallon drums of alchemist fire or acid handy to create obsticles for the enemy to deal with while your fighters chop them to ribbons. They will love you for it, and GLADLY play the roll of meat shield to you.
Hope that helps.
