I wouldn't allow it in my campaign, but I'm having a hard time understanding exactly what is happening in yours. I'm envisioning some sort of sequence like this.
Round 1) You cast Haste. (1 3rd level spell). Using your extra partial action from Haste, you cast Spectral Hand (1 2nd level spell) and lose 1d4 h.p.
Round 2) You cast a second Spectral Hand (2 2nd level spells) and lose 1d4 more h.p. You cast Brutal Seething Surge (2 3rd level spells).
Round 3) You make two melee touch attacks (at a distance) with the first spectral hand, using your two attacks per round as a 15th level sorcerer. Using the second Spectral Hand, you make an offhanded melee touch attack. Note that although the off-hand 'weapon' is automatically light, if you don't have two handed fighting and ambidexterity this will give you a -4 penalty on your first two attacks, and a -8 on your third attack. You then use your extra partial action from haste to make a fourth melee touch attack.
Round 4-5) Repeat round 3.
I don't see your combo as particularly broken (You've cast 4 spells and done 2d4 damage to yourself to obtain it, and the second spectal hand isn't helping you all that much. In fact, if I read the rules correctly, it should be hurting you.), however I do see the spell as mildly broken. The reason is that Monte seems to be assuming that a 3rd level spell is worth the ability to do 2 2nd level spells per round (once you hit 10th level) for 5 or more turns. I don't think it is. I also think that he's assuming that since it is a touch attach (a limiting factor), and has one target per touch (another limiting factor) that having (at high levels, another limiting factor) comparable damage to a good 3rd level offensive spell AND a fort save against an extra effect AND not having a save to half damage is ok. I don't.
Imagine that you cast fireball as a 15th level caster. You'd do 10d6 damage, average 35, and your foe would get a save (say DC 19) for half damage. Probably most foes for a 15th level caster would make such saves about half the time, for an average of 26 damage (or so). Fireball effects multiple targets, but for now, lets just assume it one big boss nasty you want to defeat.
Imagine that instead you cast Monte's spell as a 15th level caster. You'd do 1d8+15 damage, average 19, no save. Plus your opponent must make a Fort save (DC 19) or be dazed(!!!) for 1d3 turns, meaning that the opponent cannot take an offensive action (only defend). Worse yet, after 10th level you get two attacks per round. So right now, your mage is doing average 38 damage per round and 3 times in 4 (or so) has his opponent unable to respond for at least 1 round. And, he can do this again next round and the next and the next for 7 (and a half) rounds. Against two or fewer opponents this is not only better than fireball, this is MUCH better than fireball. Against most single opponents, this spell simply takes them out of the game. The limitation from it being a touch spell is simply too easily worked around with spectral hand.
The spell has to problems to me. First, it does exponential damage. It gives both more damage per level, and more attacks per level. Ultimately over the course of the spell the caster does 15d8+225, potentially to a single target. The ability to do this damage to a single target balances against the ability to do it to multiple targets at once. This would be sorta like having fireball have a larger and larger radius as you leveled up, or letting you have a 4th level spell that cast 1 fireball per level as you leveled up. One of the few things that makes this spell cool, is the ability to make multiple touch attacks with the same spell - so I don't want to do away with that. I'd reduce the damage either by allowing the Fort save to half damage, OR reducing the bonus damage to +1/2 levels of the spell caster (maximum +10).
But an even bigger problem is that Dazing an opponent of any level for 1d3 rounds is very nearly a 3rd level effect in and of itself. In fact, it is the equivalent of Tasha's Hideous Laughter, a 2nd level spell. So what Monte gave us was a spell that was repeatable and equal in effect to both a 1st level spell AND a 2nd level spell. I simply don't see this as being a 3rd level spell. In fact, I see it as something closer to 5th if you don't want to rewrite it (and even then it is a potential problem). Look at it this way, if it was 5th level - given the lethality you are experiencing with it - would you still memorize it (keeping in mind that it's DC would also go up)? In order to make it a 3rd level spell reduce the damage (as above), and reduce the extra effect from Dazed(!!!) to something like Dazzled 1d3 rounds (-1 penalty to attacks) instead.
You'd probably stop memorizing it then (1d8+7 damage + dazzled; good for 15 touches) except when you particularly thought it would be useful to do acid damage (or for role playing reasons), but then again, that's fine with me - since I'm not entirely happy with any spell that is so good you always favor it over core spells.
