Elemental Shield and Mestil's Acid Sheath all scale with your caster level without a hard cap, so they're always relatively powerful and always scale with the enemies, too - Death Armor DOES have a cap, but it's not a useful spell because getting hit at third to fifth level is inviting death. These spells all damage the enemy when they hit you - with DR and a Con buff, this isn't a problem. If you don't feel that's enough, Death Armor adds to it.

If you're more daring, you can load up on the defensive spells and drop Elemental Shield and Mestil's Acid Sheath. They're easily empowered and this can make up for the weak save - Empowering this spell generally ends up with more or less the same results as Maximizing it. Flame Arrow is a low-level spell that generally keeps up with your level progression, though it'll be weaker in the levels between 8, 12, 16, and 20. On the other hand, they can also blast the enemies to death.

Save or Dies are plentiful, though so are immunities, and Wizards can easily be made into a frontline fighter with the proper distribution(though you'll want to stop at level 15) of stats and the right spells. Wizards are quite powerful in NWN, pretty much regardless of what you do.