Just off how WoW spawns work, killing things fast enough increases the rate at which they spawn. This can be observed in a number of locations and situations. The point is, with a group of 25 characters, I would imagine you could find a nice plot of land to park yourself on, and just kill things as fast as they spawn. How fast this would level a raid group, I have no idea. They wouldn't necesarily even have to be raided. Just 5 5-man groups killing things fast enough to maximize respawns.
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