An old 1e trick comes to mind.
"Cursed" scrolls used to be scrolls that went off as soon as you read the title. It only worked for spells intended to affect the one reading the scroll. It was possible to use this to trigger prepared spells at a rate faster than one per round.
You can activate a single spell completion magic item in a round. But a single scroll can hold several spells.
Cursed items still exist, and can be prepared under 3e rules. They cost more than standard, naturally, but you can stack several of your "spell up" defense spells on a single page, then just read the titles one after another.
Now this is rules abuse, a gray area, and certainly requires DM approval.
The argument to allow it is simple: Spell casters who stay out of a battle for 5 rounds because they're throwing defense spells are PCs who are pretty much out of reach. The Dm's monsters can't do much to them (at least, not if they know what they're doing.)
So while this trick lets them hit the field all powered up in a single round, it also gets them out where things can happen to them in a single round.