Its called delay spell and time stop.
IDHMBWM but if I remember time stop correctly (you get seperate set or sub rounds that don't count against actual round durations), simply use every 9th level spell - lets figure you are a sorcerer with 6 spells per day +1 for ability score, with lots and lots of scrolls.
Lets figure 20 time stops to be fair.
Again IDHMBWM but figure they each net 5 actions. With each time stop partial action you cast another time stop. So you can maximize it at something like 100 sub rounds. Spread this out over 5 rounds, I think that will net you an additional 10.
So you essentially have something like 110 spells you can cast. Since you are using the delay spell the max level of spell you can cast is going to be 4th since you have used all of your 9th level spells. Assume you have two rings of wizardry (you can easily afford at level 20) so you get, what something like 18 spells of each level to cast. You will probably get 10 second and 1st level spells. So you have a little less than 60 spells to delay. Lets add in scrolls for the rest of the spells.
So lets assume delayed fireball, delayed sculpted fireball, delayed magic missile, and delayed acid arrow.
In one round this will yield you 18x10d6+18x10d6+50x10d6+10x5d4+10x2d4+5x10d6. This is a gross approximation. But just go with me for the point.
The total damage is 3430. This is to one opponent in one round. You could be invisible, ghostformed, flying, with nondetection while casting and could have scryed on your unfortunate enemy before hand. Finally, you could have contingency set up to instantly warp you to whereever after you unleash hell's fury in the form of fire acid and force.
I am sure there are plenty of mistakes here. I am also sure you would have to fairly specifically choose feats.
For the above you would at least need
magic missle
melf's acid arrow
scribe scroll
fire ball
delay spell
arcane preparation (I think that is the one that let's you prepare spells)
time stop
ring of wizardry III
ring of wizardry IV
50 scrolls of fireball
10 scrolls of time stop
Perhaps timestop doesn't work that way in which case you would have to just do it all in one round and delay anything. In this case you might as well use more powerful spells - you could still yield something like 100 or so actions.
In fact if you took energy substitution and energy admixture and had levels in Incantrix you could delay admixed spells and with haste get two per round (not including time stop) to get 10 20d6 spells off in one round and 2 20d6 empowered and maximized.
You don't even have to be 20th level for this one.
Whatever the case may be, it really doesn't matter. No one has that many hit points. Even the maxed out hit point blob will never have enough hit points to resist the smacks at this level.