It's all about initiative. Put high scores in Dex and Int (you need 19 to cast time stop).
I suggest the following:
Race: Halfling
Class: Transmuter20
Feats: Empower Spell, Improved Initiative, Maximize Spell
Dex: 18 (base) + 2 (racial) + 5 (leveling) + 5 (inherent) + 6 (enhancement) = 36
Int: 16 (base) + 6 (enhancement) = 22
Spells Prepared: 6-disintigrate, greater dispelling (5); 9-time stop (5).
Equipment: manual of quickness of action +5, gloves of Dexterity +6, headband of intellect +6, scrolls of enervation (50), scrolls of time stop (4)
Equipment value: 259,800 gp
If your DM lets you come in with spells pre-cast, drop a time stop for a twice-Empowered, Maximized cat's grace. It lasts 20 hours and gives you 7-10 points of Dex (increasing your final score by 1d4+1, since it doesn't stack with the gloves). If he doesn't, you still have Initiative +17.
When you go, cast time stop as needed; with 4-5 prepared and 4 scrolls, you should get about 20 rounds to unload on the party. Pick any nice spells, but be sure to have some that don't allow saves for the monk and group members with good saves.