Addendum: The author of the original Locate City nuke trick has provided the following updated version of this trick.
Trying again after reading up more about it on the WotC boards. For this tactic, you just need to be a Human Sorcerer 6 with the Metamagic Specialist variant in exchange for your familiar. Take two flaws so that you can take Arcane Thesis (Locate City), Born of Three Thunders, Energy Substitution (electricity), Explosive Spell, Flash Frost, and Snowcasting your feats. Make sure you have Locate City on your spell list, and pack along several scrolls of Lesser Globe of Invulnerability.
Cast Lesser Globe of Invulnerability from a scroll so you'll be immune to the havoc you'll be causing. Now grab some ice from a freezer as a move action and spend the entire next round casting Locate City, spontaneously applying Flash Frost, Energy Substitution (electricity), Born of Three Thunders, and Explosive Spell in that order. That'll use up a 3rd-level spell slot due to Arcane Thesis, but you should have four 3rd-level spell slots assuming you have 18 Charisma. Because of Snowcasting, the Locate City spell gets the [cold] subtype. Flash Frost makes it deal 2 cold damage to everyone in the area. Energy Substitution (electricity) changes the spell's subtype to [electricity] and turns the damage into 2 electricity damage. Born of Three Thunders then makes that 1 electricity damage and 1 sonic damage, and forces a Fortitude save against being stunned as well as a Reflex save against being knocked prone. Explosive Spell only affects spells that force a Reflex save, so we've got that covered.
Locate City affects a 10-mile/level radius, centered on you. At caster level 8th, thanks to Arcane Thesis, that's an 80-mile radius, or a 422,400-foot radius. Anyone who fails both the Fortitude save and the Reflex save, which should each be DC 15 assuming you have 18 Charisma, will be pushed to the nearest edge of the area, taking 1d6 damage for every 10 feet moved. If someone was right in front of the caster and failed both saves, he'd be hurled the full 80 miles and take 42,240d6 damage (average 147,840 damage). That's going to be a very forceful and painful trip indeed. Once you cast it, all structures in a 80-mile radius will be utterly destroyed, and approximately a third the population will fail their saves and violently get killed. Now that I think about it, the ground would take damage and auto-fail its saves as well, so you'd end up with a crater 80 miles deep. Then after a round of being dazed from Born of Three Thunders, you cast it again and again and again, putting up a Lesser Globe of Invulnerability as needed.
That is what you can do at 6th-level. Now we'll need to go Incantatrix for the next 10 levels, but unfortunately, we don't happen to have Iron Will. However, the retraining rules from the Player's Handbook II lets us switch out a feat of our choice for another feat before leveling up, so we're free to switch out Born of Three Thunders for Iron Will right before we take Incantatrix. We'll get BoTT right back since Incantatrix gives you a bonus metamagic feat at 1st-level. Right before hitting Incantatrix 4, we'll switch out Energy Substitution for a non-metamagic feat, and then get the former right back as a bonus feat. Before hitting Incantatrix 7, we'll do the same by switching out Explosive Spell for another non-metamagic feat of our choice, and then get Explosive Spell back as a bonus feat. Once we reach Incantatrix 9, we'll simply take Heighten Spell as our bonus metamagic feat.
As a Human Sorcerer 6/Incantatrix 10/Stormcaster 1/Archmage 3, this is what your feat list should look like: Arcane Thesis (Locate City), Born of Three Thunders, Draconic Heritage (any electric), Draconic Power, Energy Substitution (electricity), Explosive Spell, Flash Frost, Greater Spell Focus (divination), Heighten Spell, Iron Will, Skill Focus (spellcraft), Snowcasting, Spell Focus (divination), Spell Focus (any).
A Snowcasted, Flash Frost, Energy Substituted (electricity), Born of Three Thunders, Explosive, Heightened to 9th-level Locate City with Arcane Thesis and Improved Metamagic will take up but a 6th-level spell slot. Fill out your lower level slots with less metamagic'd Locate City spells. Assuming you took Spell Power as your High Arcana three times, you should have a caster level of 28th (20 base + 3 Spell Power + 2 Storm Spell Power + 2 Arcane Thesis + 1 Draconic Power), so your devastating spell will now cover a 280-mile radius. Assuming you have 34 Charisma (18 base + 5 stat boosts + 5 stat tomes + 6 Cloak of Charisma), the Fortitude and Reflex save DCs should each be 34 (10 + 9 spell level + 12 Charisma bonus + 1 Spell Focus + 1 Greater Spell Focus + 1 Draconic Power).
1d6 damage per 10 feet moved. If you eat the full 280 miles, that's 147,840d6 (average 517,440 damage). We'll take the average for the purpose of calculations. If you somehow have regeneration 1, you'd fully heal the damage in 862.4 hours or 35.93 days. The tarrasque, with regeneration 40, would heal it in just 21.56 hours.
Arcane Thesis, Flash Frost: Player's Handbook II
Born of Three Thunders, Explosive Spell: Complete Arcane
Draconic Heritage, Draconic Power: Races of the Dragon
Locate City: Races of Destiny
Incantatrix: Player's Guide to Faerun