There's also a giltch that I found at http://borderlands.wikia.com/wiki/Mad_Moxxi%27s_Underdome_Ri...
that will help you skip some rounds entirly. Using this method I completed Angelic Ruins in two hours. Credit goes to who ever posted this on the Borderlands Wiki.
"There is in fact a way to skip a few of the rounds.
What you will need:
1. Eridian Shotgun(Thunder Storm)
2. 2 players(one must be a low leveled character, the other a high level)
1. Run through the first 4 waves(The round skips can only be done at the boss wave)
2. Before the boss wave starts, position yourself aiming at the center of the stage(this can be marked by the ramp(the middle of the three sections of the ramp)
* Immediately upon the boss spawning, shoot and kill him with the thunder storm
1. Do not kill the henchmen yet.
2. If you see the curtains close while the henchmen are spawning, this has been executed correctly, otherwise you will need to wait until the next round.
3. After the curtains close, you will need to wait a few seconds until a countdown timer starts from 10.
4. After continuing out of the end of round stats, kill those henchmen.
5. Wave 1 of the new round will start.
6. A few seconds later, the countdown from 10 will start.
o After the countdown you will get the end of round stats. Continue, and the new round will start.
* The boss needs to die before his henchmen spawn. ie. Nine Toes would need to die before his dogs spawn
o You will still need to kill wave 1 of the prior round that you skipped.
Why do I need...
The thunder storm - Because regular bullets will only do 1 damage to the enemy, whereas the thunderstorm does a great deal of elemental damage fast enough to take out the boss.
To wait before killing the henchmen - The programming that was done appears to be that if there are no enemies the boss wave on the field after the first one appears, end that round. By killing the boss fast enough, you make this true. The henchmen still spawn, making it so that when you kill them after the next round starts, this is true again.
This basically saves you 4 rounds each time, as you will still have to fight wave 1 of the previous round. It will still save you the additional time, as wave 1 of the next round will spawn while you are still fighting that previous wave.
Note: Some boss waves this will not work with. Their spawn time is delayed or they have extreme recharge on their shields upon spawning. So basically you'll be able to get, within the 20 rounds, 3-5 of them skipped, which can save upwards of 1-2 hours depending."