I would probably use multiple Divinations and Communes to narrow down what tactics to use, then I'd scry out base defenses with Scry or Greater Scrying.
Then buff up with everything you have, get several Planar Allies and use a Limited Wish to gain +7 on your next initiative roll.
Teleport in and Harm the warlord, then kill him and grab his corpse to prevent Raise Dead and such. Once the warlord and his bodyguard are dead, Windwalk or fly up and unlease all you tactical nuke spells on his army. Sunburst, Firestorm, and Storm of Vengence are my personal favorites, followed up by several hits from a Wand of Fireballs.
For your Planar army I recommend Planetars for sheer melee hobgoblin whomping and Ghaeles for support, as they can cast as 14th level clerics, have a Gaze attack that cleans out low-level goons, and due to their domains can each have a Changestaff Treant.
A prepared 16th level party that's willing to call in all the chips can take out just about any threat in a few quick minutes of destruction.
Of course, it's even worse for evil characters, as a CR 18 Balor has only 13 HD. Imagine how much havoc 8 Balors loose in a city could cause.