Slight Star Trek TNG reference there

I'm level 15 now now, 13 wiz/2 rog Human Palemaster. And what I am noticing now is how many hit points a lot of these standard enemies have - Ogres and Giants for example. Until recently I could boost Ice or Fire enough that I could pick these guys off one by one with Scorching Ray or Frost Lance, so save sp (8 and 10sp, respectively). But now, unless it crits, those spells do not kill in one shot. In fact several enemies now survive TWO shots, so sp is draining fast.

I am quite survivable, do to Wraith form + Displacement + Auras, so its not like I'm scared of dying. But the defenses consume sp too. What I'm seeing now is my sp bar going down FAST just for one group of 5+ enemies. I need advice on how to kill groups quickly but also efficiently with this character.

I have over 1200 sp right now (34 INT and a lot of sp boosting enhancements and a + 175 sp item), so I'm pretty much maxed. I do have the Maximize feat too, if needed. I usually run with a level 15 cleric just for Divine Vitality III (I think that's what it is called.)

Here are my options so far:
1) Keep using Scorching Ray and Frost Lance, with Fire and Ice VII and max Fire/Ice gear. This is getting inefficient, and I also have to cast at least one defensive spell before they all die.
2) Maximized Freezing Sphere or Delayed Blast Fireball. This frequently kills most/all of the group (if I can get them grouped), but it costs 50sp to cast.
3) Finger of Death - 90% success rate right now, but I think it costs 40sp and is one target only. I usually only use this on orange/red names.
4) Sphere of Dancing - Awesome spell, and usually works, but it just controls them, doesn't damage. Seems like a better spell for grouping than soloing. It costs a hefty 40 sp too.
5) Shield damage? Haven't tried this yet, but maybe with both Fire Shield and Ice Shield, + Death Auras, just let them hit on me till they die? But this would consume over 50 sp per group also, and I'd still have to deal with the ranged enemies.
6) Invis? Haven't tried this yet either. Basically, stop fighting all these groups in some way? (which results in less xp I think.)

So basically I was breezing through fights fast, and now they take a while, and if I can't find a shrine somewhere then I can get into trouble. This is where being a sorcerer (guessing here) really shines with their big pool of sp. How do you experienced wizards deal with groups in an efficient way?

Thanks for your help, as always.